Real question is whether you want a graphics card that is massively overpowered for that system but will be ready to fit in around a new (more powerful) PC in the future, or if you want a graphics card that'll fit with the rest of the spec and save some more money towards an upgrade later?
Put in anything above a HD5750 / GTS450 and you're likely to be bottlenecking to a degree: after all, it's a low power dual core processor you're dealing with here. but as Kalniel says if you go for something higher up the scale like a 5850, GTX560, HD6870 then you'll get as much performance as a 2GHz dual core can provide now, then a boost in performance *from the same graphics card* when you get a more powerful rig later. On the other hand, you could save money now by getting a lower spec card (perhaps a second hand 9800GT / 8800GT?) so you'll have more money to put towards whatever the latest & greatest is when you do get round to building that new rig.
And that decision, I'm afraid, is up to you
EDIT: just to say that if it was me, I'd be getting
one of these