If you'd like any beefier card, then you should start looking up for PSUs (power supplies) also.
What's your current monitor resolution? Are you planning on a 24" or bigger monitor?
On a wild guess, if you're on an 1650x1080, your bottom line should be something in the lines of a 6850 or a GT550ti to get all the eye candy in modern games (or most of them). Higher than that, you'd have to start looking at a 6950 1GB or a GTX560ti (GTX460 1GB too).
The PSUs that can accompany those cards are 500W and above. Don't have models in mind, but I've seen Corsair and CoolerMaster ones running some-what cheap and with good quality.
EDIT: Forgot about Antec; this one specially: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035