Something like a Geforce 260 series or ATI hd5700 series (or better) graphics upgrade would probably give you the best bang for the buck right now. Pick a card with an eye on recycling it in a new build later on.
You could move up to a 3 GHz or so processor (dual or quad) but that's 200 bucks you would not be able to use on a new build. (assuming a new build would be i5, i7 based)
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