Stm_64 we need more information that what you have provided. Like what psu you have, what your current card is etc.
Now if it were I, i'd go ati/amd (i'm a fan boy yes) and tell you to pick up a the best card you can, probablly a 6950 or a 6870 for your price range. I do recomend staying away from the Asus DC II's though, not that its a bad card, but taking up 3 slots is just terrible. I know because I have one.