I have 3 GTX570 in triple SLI. I plan on using nVidia surround to power 3 27" monitors at 1080x1920 (portrait).
However, I'm curious if the 1280mb vRAM each card has will be a limiting factor? If so, anyway around it? Considering the cards still cost $300 and the monitors $300 I'm not going to upgrade for a long while...
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I wouldn't be comfortable with 1280mb, I really think 2gb is the absolute min for 3 1080p screens either portrait or landscape. For future proofing, I'd recommend 3gb but 1280 is a tad low in my opinion especially since BF3 in a single 1080p at ultra needs like 1.5 gb, let alone 3 screens.
If you already have the 570s, then you might as well try it, but your vram will definitely be the limiting factor.
Skyrim is going to be a major problem. Even without mods, 1gb is about how much it uses on a single 1080p screen. I think the only way is to turn AA off for skyrim. With mods, I've heard 2gb is easily reachable. I think with a 3 screen setup, your going to be disappointed.
Witcher 2 is going to be much easier to run than Skyrim and I want to say you should be fine.
Diablo 3 is going to be the easiest game for you to run maxed out so that should be fine.
One caveat, just because I've seen 2gb of vram used doesn't mean it's a requirement. With a 320bit bus speed, which is actually very good, ir could help compensate.
At the end of the day, with almost 750 dollars worth of graphics cards, you're not going to get the best performance, but it's still not going to be terrible.
The last idea I have is to sell your 570s and get a 670 4gb or a 7950 3gb cards in SLI/CF. That should relieve of any vram issues.