1) Read the new article at Tomshardware on micro-stuttering.
2) New AMD cards in Q4 2011
3) New NVidia cards in Q2 2012
A single GTX570 maxes out pretty much every game. By the time you need more than that you can upgrade.
The new cards from AMD and NVidia will use a 28nm die process which should use roughly 2.5x less power. This means the cards will be MUCH QUIETER. They will also be able to make better performing cards than they have now (heat is the limiting factor).
Currently they have to use multiple GPU's (on one card or having multiple cards) however stuttering becomes an issue.
I'd wait for either the new AMD or NVidia cards, and forget about multiple GPU's.