Biggest graphics card I have seem in a Shuttle is an old 9800XT in an old AGP+Athlon64 box. The guy had to dremil vent holes in the side of the case around the video card fan to get it stable, but the PSU was fine.
Try feeding your intended system into the PSU calculatr and see what it comes up with.
In terms of future upgradability, AMD have a far better track record than Intel where although they keep calling new CPUs socket 775 each new CPU requires a nwe variant of that socket to work.
AMD have stated that their next gen AM3 CPUs should work in AM2 sockets with a BIOS update as they will be backwards compatible with DDR2.
Personally I wouldn't spend that much on a CPU, but perhaps that is just me.