You guys do realize that the 7950 doesn't use SLI by itself right? You don't have to have a SLI motherboard for it, just need a PCIe slot.
Reasons to get 7950 over 7900GT SLI:
7950 runs faster
Only uses 1 6-pin power connector
Only uses 1 PCIe slot
Can upgrade to Quad SLI once forceware drivers (but thats a waste of money IMO)
SLI 7900GT and 7950 actually are pretty good, the 7950 runs more then 100% faster in FEAR w/ 1600x1200 (75fps vs 35fps) and even 2048x1538 (it runs at 75fps!)
The 7950 once agains is running just around 100% faster then the 7900 GT in HL2 Episode 1 in 1600x1200 and 2048x1538.
It also runs 100% faster in BF2 w/ 2048x1538 4xAA.
The 7950 and SLI 7900 GTs are the only multi-GPU that I support and think is worth the cash... but I only think it's worth it if you have a really high resolution monitor.