Well, what do you do with your computer? I'm going to assume you're a gamer at least with that rig.
Newegg.com - MSI R4890-T2D1G OC Radeon HD 4890 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - OC Edition - Desktop Graphics / Video Cards
Factory OC'ed 4890, cheaper then any other 4890, and free shipping. The Nvidia GTX 275 is the competitor to that card. Both perform very well. Some will prefer the GTX 275 and others the 4890. Either is a great choice.
What type of power supply do you have? If I had that computer and was looking to upgrade on a limited budget I would go with a 4890 or GTX 275 and look into getting a new power supply if it's a not a solid one.
EDIT: I should also add, how much are you thinking you would want to spend outisde of your video card budget? You can get a pretty nice performance boost with a newer CPU and motherboard on top of the video card.