Hp a6421 PSU Upgrade

By the way my specs are amd athlon 64x2 4400 2.30ghz, 360gb HD, 3gb RAM, vista and will be hoping to put in a gt 9600

Looks like thats a ATX desktop? You might want to take out the PSU and measure it to make sure its ATX. If it is then your in luck, if its something like BTX like Dell makes then amybe/ most likely yes tho. I'm not sure if that 4400+ will create a bottleneck for that card.