Yeah I would agree on changing out the PSU for something better, if your really looking on future proofing your graphics card. There really isn't a way of doing this as every next gen of cards that come out are going to be better then the last one you just bought, I would just focus on something that will let you be able to play what you want to play for a good period of time and be good enough to play the newer stuff at a decent resolution on decent settings if not already the highest you need. Since I myself am a nvidia fanboy I would suggest something like a GTX 460,
NCIX.com - Buy EVGA GeForce GTX 460 Fermi 675MHZ 768MB GDDR5 Dual DVI Mini-HDMI PCI-E DX11 Video Card - 768-P3-1360-TR In Canada.
You can get it under $200 with the mail in rebate, however the new ATI series of 6850's can be unlocked to run at 6870 clock speeds which is a better bang for your buck, so if you can find one on sale for around 250-270 pick it up and look for the unlocking thread here that lowfat made.