^In order to upgrade that computer, you'd have to pretty much gut it. New motherboard, new chipset, new ram.... sticking a new video card in a five year old machine really isn't worth the money, unless it's a $60 card and you're only doing it because the old one died. Your processor will be a tremendous bottleneck.
Really, if you're going to put $400 into a five year old machine, spend twice that and get a rocking i5 machine that will last you another five years. Especially if you're compiling video and stuff, you'll notice the difference in speed.