The XTX has GDDR4 as far as I know, it will be hard to push the GDDR3 to 2.2GHz. My point is that now, you can still get at least 250$ for it.
I have a 8600GT. When Chrisis will appear, and with the 8600GT I'll be able to play it, there's no way I'm gonna pay more than 150$ for a card that can't. It is true that in Dx9 7900GTX, x1950XT and others pawn my GFX card, but I don't care about that, and there are many that don't.
IMHO a 8800GTS 320 MB will do well enough in Dx10 to be able to play at 1440*900 max details and maybe some 2xAA/8xAF. It seems a fine deal to me, and it's even better if you have the money to get a 2900XT or a GTX.