Theres no technical reason why you cant go as far as you want then in terms of upgradging your graphcis card. You could put the worlds best card in there if you wanted but yes, it would probably be limited by other factors.
I would consider what you use the machine for, what you have to spend and then pick a card that fits in with the rest of your machine.
7800GS has been mentioned, and wouldnt be a bad option.