your cpu won't be able to take the advantage of today's card (not to mention that your psu may not be able to handle the higher end cards) for your budget, I think go for 6600 or 6600GT range or the ati counterparts.
I think even the couple of gen's cards such as ati's 9800 would be much improvement over your current card. I think don't spend more than 100 and save that extra 100 for future upgrade, such as ram or new pc.