simon12 :
+1 to the above 2 x 580s is more than any game can use at 1920x1080 one is overkill. I suggest you get one and get the second in a few years time. As for bottleneck it doesn't really matter if your getting double the FPS you monitor can display.
Pretty much. The resolution is the largest bottleneck to a high end GPU right now, hence why I have never gone CFX or SLI. If you want to do a multi-monitor setup, like 2 or 3, then SLI or CFX is worth it. But with a single display most current GPUs from about the mid high end (think GTX570 or HD6850) will easily max the games out for that with no issues.
Save the money from the second GTX580 and kill off one of the largest bottlenecks in your system, the HDD. Buy a nice SATA6 SSD and watch Windows load like no other. Just make sure to go with Windows 7. XP is a SSD killer, no TRIM support, and Vista, well Vista just isn't 7.