At the momment, in all honesty, it really comes down to which comapny you like more.
At the momment, ATI leads the benchmark scores withtheir new line of X1900 series, but something tells me that it wont be long before NVidia come up with something to take over, and then, once again, ATI will have a comeback and so on and so forth.
I just bought the X1900XTX which is the best single card on the market and could not be happier, but with great power comes great debt
If you give us a picture of your price range, we could make some suggestions for you, though you said you had AGP, and if your PC is getting old, a top of the line AGP card may not be able to be used how it should.