Understanding SLI/Crossfire

[Computer-Ed]

Okay lets say we have 2 8600GT cards and a single 8800GTS 320 meg. Would the 8600GT SLI be faster? Would the two 8600 be equal to having a single 512 meg video card for memory?

The same with ATI, would a 2900XT be faster than 2x 2600XT?

[Yellowhello]

Two 256MB WILL NOT equal one 512MB. Check out this link: FAQ

[gvblake22]

This review has some benchmarks comparing the 2600 cards with 8600 cards in single and dual card configurations, but does not benchmark any 8800 or 2900 series cards with it.

As for your question about the two 256MB cards, thinking it will be like having a 512MB card is not a very accurate way of thinking about it. First off, just because "card A" has 256MB and "card B" has 512MB does not mean "card B" will perform better than "card A", it all depends on the architechture and whatnot. Second, if you were to compare to identical GPUs (two 2600XT's for example) but pair one with 256MB and the other with 512MB, there would not be a huge difference compared to running two smaller memory cards in SLI/CrossFire. Case in point is the 8800GTS as it comes in both 320MB and 640MB flavors, but two 320MB 8800GTS cards in SLI will easily outperform a single 640MB 8800GTS.

EDIT:
I just found this review at Hot Hardware that compares all the cards in question except the 2900XT. But we already know how that card compares to the 88000GTS, so you can fill in the blanks...

[[Dr. V]]

All very true.

You have to remember that the quantities don't add up. It's the shear processing ability that is pretty much doubled.that brings insane performance boosts. Like blake mentioned 2x8800GTS 320MB in SLI is FAR better than 1x8800GTS 640MB. Because you have so much more processing power, the RAM matters less and less if you wanna look at it that way.

In terms of 2x8800GTS versus 1x8800GTX, it's likely that it will perform very very close to par if not better.

[Computer-Ed]

Originally Posted by gvblake22

I just found this review at Hot Hardware that compares all the cards in question except the 2900XT. But we already know how that card compares to the 88000GTS, so you can fill in the blanks...

This numbers are just scarey. A 2600XT in crossfire is needed to keep up with a X1950pro in single card. That is NUTS! Especially since a number of ATI card makers are claiming the 2600XT is equal to a X1950Pro.

From the benchmark numbers it would appear that the 8600 GTS in SLI will run right with an 8800GTS 320 in single card mode. However with 2 8600 GTS costing around $320 and a single 8800GTS 320 costing around $280 it would appear SLI and Crossfire are really a waste of time unless you go to the high end.

Even then it is not worth the cost unless you have a huge monitor to run crazy high resolutions.

[gvblake22]

Originally Posted by Computer-Ed

From the benchmark numbers it would appear that the 8600 GTS in SLI will run right with an 8800GTS 320 in single card mode. However with 2 8600 GTS costing around $320 and a single 8800GTS 320 costing around $280 it would appear SLI and Crossfire are really a waste of time unless you go to the high end.

Even then it is not worth the cost unless you have a huge monitor to run crazy high resolutions.

Now you've got the idea!
SLI and CrossFire are really just creative marketing schemes to get you to spend twice as much money. You are almost always better off getting a single higher-end card than you are trying to run two mid-range cards in SLI or CrossFire.

[One4yu2c]

Originally Posted by Computer-Ed

Even then it is not worth the cost unless you have a huge monitor to run crazy high resolutions.

That's pretty much always been, and will continue to be, the bottom line regarding multiple videocard setups.

[Stormcrow]

There are two advantages to SLi or Crossfire, and that's to us budget builders who work with a strict amount of cash. Buying a single 8800 for $3-400 is easily a daunting task, I've had to save and plan out a budget for a few months now. However, it would be ALOT easier for me to spend 130 on a 8600 GT now, and then add another in SLi to boost performance a couple of months down the road. While not being able to match a single card, I'm able to get decent performance now, and then add to that performance later on as time allows. But in some cases, such as the 8600 being so far below the 8800, it might be better to just put it off until you can afford the high end like I have.

Another is that next year when the G90 or whatever comes out and fills in the 500 dollar top line bracket, I'll be able to pick up another 8800 on the cheap through eBay or Newegg, and add that much more to my performance until I can see how the mid-range of the new cards preform and for the high end to come down in price like its doing now. The same is true all around for SLi and Crossfire.

Not to threadjack, but I heard that the firmware for the cards will effect how their connected, and if you get one now, and then one months down the road, the firmware may have changed to the point its incompatible. If thats true, it makes my whole statement moot.

[gvblake22]

Originally Posted by Stormcrow

Not to threadjack, but I heard that the firmware for the cards will effect how their connected, and if you get one now, and then one months down the road, the firmware may have changed to the point its incompatible. If thats true, it makes my whole statement moot.

I've not heard of that yet. I remember when SLI used to be only possible on exactly identical cards from the same vendor. Now you can mix manufacturer types and it is OK, so I don't see how having a different firmware would make a difference...

[[Dr. V]]

Wicked. 900th post.

Firmware is an odd thing to call it. He may very well be referring to the card's BIOS.

WHAT? VIDEO CARDS HAVE THEIR OWN BIOS?

Yes, they do. And just like motherboards, the BIOS greatly affects performance and compatibility. Actually, you should upgrade your BIOS especially when you're about to add another card for SLI, if not once every month or two (meaning look for an upgrade, doesn't mean there'll always be one). How do you think people with a 7900GS basically "transform" their cards into 7900GTs? There is a specific chip that they produced before the current ones in the 7900GS that are unlockable to 7900GT via its BIOS.

Also, blake brings up an excellent point. You had to have two exactly identical cards. If one was an XFX 7900GTX XXX edition, the other one had to be as well, or SLI wouldn't work. Now, with the new boards and BIOSs of both the boards and the video cards, you can mix and match. Also, for many setups, the SLI bridge isn't necessary anymore, although I personally recommend using it if you're going to have an SLI setup.

2x lower end vs 1x higher end will always be a debate, and will never have a conclusion. Twice the raw processing power is awesome, but what if for the same/lower price you can get even more processing power in one card? And what's the advantage of two cards if it isn't double the memory? What's SLI best for?

All these questions and more have been touched in this thread, and will continue to be asked with no solid answer. It comes down to personal need/opinion. It's hard to say if 2x8600GTS will outperform 1x8800GTS. For higher resolutions, memory is the most useful thing you can have...but is this true for an SLI setup? Possibly not. But every answer is maybe, or possible, or perhaps. Never gonna change.

The best thing is to give an example, like tell us 2x A vs 1x B and I'm more than positive we'll be able to suggest what the best is for your setup.