To be the honest buying AMD now is crazy, it's not just that they are slower - they are old tech. Games like Sup Comm really benefit for dual core.
Hogwash.
1. AMD (Ati) video cards are just fine. DirectX 9 cards will be servicable for two years at the very least, just like DirectX 8 cards were usable years after DirectX 9 was introduced. Hell, you can play Half Life Episode 1 with a DirectX 8 card and it's almost indiscernible from the DirectX 9 codepath.
There is a huger install base of DirectX 9 cards and developers can't ignore them unless they want to go out of business. Pretty much every game in the next 2 years will have a DirectX 9 path.
And for the record, Ati cards are not slower in general, yes the 8800 is the fastest videocard out there but there are price points where Ati cards are still king, like the $250 X1950 XT.
AMD CPUs are fine too, at resolutions above 1024x768 the videocard will be the bottleneck, not the CPU. I'd personally get a Core2 if you're buying new, but an Athlon X2 is still a great gaming cpu.
2. Dual core is far from necessary, the few games that do utilize it so far don't make a 'must have' difference. As far as supreme commander, we'll see.
But to answer the OP's question... I'd wait for the 320mb 8800 GTS. The 7900 GTX is way overpriced, the $250 X1950 XT will beat on it.