amd= better performance per watt/dollar, cooler temperatures/less noise, more frequent driver updates, excellent multicard/multi-monitor support.
nvidia= more heat/noise, higher power usage, more overclocking room, less frequent driver updates. gimmicky physx and cuda stuff you don't need to worry about or base your decision on.
Toms has a monthly article that is very good for helping people select video cards
http://www.tomshardware.com/reviews/best-graphics-card,2964.html
Highly recommended it. Pay attention to the Hierarchy at the end....i have an amd 5870...anything on the same level or better and you'll be very happy assuming your cpu/ram are worth a crap.
like the guy said give us the rest of your specs.