Wow, a lot of disinformation since my last post. Let's clean it up a bit:
-The dual GPU Nvidia card is a GTX 590, NOT 595
-The 6970 competes with the GTX 570 NOT 580, which stands alone as the fastest GPU.
-Overclocking might void your warranty, but unlikely due to it being so difficult to prove that the damage came from overclocking. In fact most video card manufacturers include overclocking software with their cards. Nvidia itself offers overclocking software as well.
-No one should suggest a SLI combination without first checking to see if you have a SLI capable motherboard and a power supply to support it. Its a good point, though, a pair of GTX 560's is the way to go if you are able to spend GTX 580 money, and have the motherboard and power supply to support it.