Originally Posted by friendlyarrows
Well it really boils down to One card, and the memory bus on that card. Crossfire will not give you extra video memory, it is set up to only use the first cards memory. Whats more is that when doing multiple monitors it would require more access to the video ram, meaning a wider memory bus would be very important for this. Take that into account on Ati's cards, where most are 256 bit memory buses, Nvidia's cards are 300+ bit buses. With all that being said I personally think your best bet would be to reasearch memory buses and compare performance of them at higher resoultions, that should help your decision.
Memory bus width is an arbitrary number, like GhZ, and doesn't matter by itself. What does matter is memory bandwidth
, which is a factor of both the memory bus width and the memory speed. The 5xxx series isn't necessarily bottlenecked by memory bandwidth, so this isn't a major concern.
On to the answer to your question. The above response was right, multiple cards in crossfire don't give you additional video memory, the VRAM is just mirrored over each card and the rendering jobs are split between the cards.
Resolution increases (even major ones such as eyefinity) also don't affect GPU memory use linearly, which is something most people don't realize. 5760x1200 doesn't even use twice the VRAM of 1920x1200.
Another interesting fact is that Antialiasing VRAM use scales per pixel, so AA has a much higher VRAM use at 5760x1200 than 1920x1200. At high resolutions, AA is simply a killer.
In normal use scenarios, you should not be worried about running out of 1GB VRAM. 5760x1200 with 2x AA uses less VRAM than 1920x1200 @ 8x AA. Worst case scenario, you have to dial down the AA levels on your eyefinity games (which you will be doing anyway because of decreased performance at uber high res), but the res increase alone doesn't have much of an impact on VRAM use.
edit: As for performance, as long as you overclock your 5850 well, eyefinity shouldn't be too much of a performance problem in current gen games. I'm sure future DX11 games with tesselation use will require more power, but my single 5850 (950/1135) runs Bad Company 2 in 5760x1200 on everything high (no AA) @ 30-50fps.Edited by mechtech - 4/23/10 at 1:49am