should I do it? 2900XT for 8800GTS 640.

[nicepun]

afaik the 2900 has the same hsf mounts as the x1800/x1900 series, so u might find a cooler for cheap.

However, the performance increase will not be significant, unless you're running and gaming on vista

i'd recommend to wait for prices to drop on the 1gb ver of x2900

HSF is different and it's not as loud (specially the people that do not own one) make it out to bed. Your PSU might be enough but don't expect high OCs. And yes, wait for the 1GB version.

No option, you shouldn't.

Why trade up for a card that you'll lose performance when you kick on AA, has worse image quality, runs louder, and eats more juice?

It doesnt run louder, it doesnt run hotter, it does eat more juice but there isn't a stardard and it's like you'll have go out and get a 1KW PSU. I owned a 8800GTS 640MB and although it gave more fps according the game benches (i.e. FEAR) the game ran just as smooth with the HD2900XT and I couldn't tell the difference or even feel the HD2900XT slower. And if you like to 3DMark benching, the 8800GTS 640MB can't touch the HD2900XT.