The video card doesn't affect the actual rendering, although it determines how smoothly you can move around the camera while editing. If you want the best rendering speeds, two dual cores would be optimal, but get at least one (would highly advise waiting for Core 2 though). Performance in most modern games, on the other hand, is almost completely dependent on the video card(s).
So yeah, I think it's possible to get top notch gaming and rendering performance in the same system if you're ready to spend the money, although maybe not overall 3D app performance (the workstation cards are much faster than consumer cards at that, but suck in games).
If you're planning to use SLI, try all the games you intend to play on your brother's system and make sure everything works to your satisfaction. SLI is wonderful when it works, but it currently has
major vsync related problems in most games (that go beyond the basic tearing). I finally got fed up with that and ditched my SLI setup last month.
If you're willing to tinker a bit, avoid the 7900GTX, as essentially all 7900GTs can be pushed beyond GTX speeds with a very simple mod.
Also, there seems to be some widespread problem on the current batches of 7900 cards, with a large number of people getting defective ones out of the box. It's bad enough that some hardware sites and card manufacturers have acknowledged it, so make sure you buy from some store/company that will let you replace it quickly if needed.
Core 2 Duo will be out by end of June and G80 according to rumors will be pretty close as well, R600 on the other hand is most likely released for christmas.
Actually, the general consensus around various forums is that they are both very unlikely to come out before the end of the year (possibly even later), going by Inquirer speculation and the complete lack of any leaked information. The 7950GX2 is yet another mid-gen refresh intended to hold over until then and ATI should have a speed bumped R580 out in September.
