IMO, if you want to game at least decently, a 9800GT or a GTS250 (lower power on the GTS) would be better, or go with ATI 5750 like dark_lord69 linked, which has the added benifit of DX11 support.
Although, 450W PSU is only good if it's a quality one. Do you know the brand? (open the case up to check)
EDIT: I should mention that you should also seriously consider a 2x2gb RAM set. I use about 870mb RAM just having Windows 7 running, and I've had games eat up 2-3gbs on top of that.