The two GPUs on the 5970 are linked via Crossfire. You can apparently flash the BIOS on the 2nd card with the Master's BIOS to get them to operate independently, but I'm not sure how good of an idea that is.
I talked to a guy with a 5970 quite a bit about 7 months ago, one of the threads is here:
http://www.dual-boxing.com/showthread.php?t=30074. He found that the Crossfire support did not help at all - the first GPU would be pegged at 100% and the second wouldn't be doing much. Crossfire and SLI are designed for playing one instance of a game with extra power, and at least one of the two only works in Full Screen mode (which is an exclusive thing, only one program can be in Full Screen mode). Neither of them are useful for multiboxing on the same PC.
Anyway, I don't recommend getting the 5970 because you have to flash the second GPU with a different BIOS to make the 5970 useful without Crossfire (which is not useful for multiboxing). Unless they specifically support doing that now.
You can use multiple video cards, but most people are completely clueless as to what kind of problems that actually introduces. Changes I'm making for ISBoxer 38 should help a bit. But basically, if you use multiple video cards for multiboxing, you are probably going to run into performance drops before you run into performance gains. The crossfire/SLI thing is the first one. Then there's the thing where if you move a game window from a monitor on one card to a monitor on the other, your FPS drops to like 1 until you move it back. To avoid it, the game needs to be told which monitor to host the game with and then it needs to stay on that card the whole time (on windows XP, you can't even move it between monitors on the same card). If it's told the wrong monitor, it'll be slow right from the start just like it's been moved to the wrong monitor (because it has). And then there's the monitor selection not being in the expected order sometimes, so people think they picked the right one but find out from trial and error that they didn't.
Higher performance
can be had from two cards running instances separately on different monitors (kinda like adding more CPU cores) but frankly, it's much
easier if you only have one video card. With ISBoxer 37 you'll either have to disable window swapping to use multiple monitors, or manually adjust the swapping behavior to swap on a per monitor basis (there's a thread where I explain how to do that with some Mapped Keys, but it's not something I'd recommend every new guy who wants to use multiple monitors should try to follow...).
With ISBoxer 38, I am currently developing a revamped Window Layout editor, which will specifically address some of the issues with using multiple video cards. It will offer built-in support for per-monitor window swapping, and it should automatically select the correct monitor for the game. The wizard should also have some additional choices for Window Layout styles with multiple video cards.
So in short... if you want good performance and have it easy to get, use one powerful video card (without a second one linked by SLI/Crossfire). If you want better performance, you can use multiple video cards with multiple monitors, but you will have to give up swapping windows into the same main area and depending on how complicated you want to get, it's going to be more complicated to get it working. I run all my 5-man WoW teams on a single GPU, currently a 5870 in my laptop but my desktop has an nvidia 8800 GTX. It had two, but I removed one because I don't use it and I like being able to swap all of my windows into the same main area without a performance penalty. I typically get 40-60 fps on the main window (limited to 60 max) and I limit them at 15 in the background.
Mon Dec 13, 2010 7:04 pm 