I can't speak for the AGP version, but I went from a X850XT Plantinum Edition to a X1950pro, and there is quite a difference. I would highly suggest thinking about the 1950pro if you are any kind of gamer, and you can swing the extra cash for it and a power supply. But then, if your cash is limited, the 2600
XT that has been mentioned is a good choice for the money.
Too bad you don't have a PCIe slot, I have that X850XT PE sitting in the closet, ran it only for about 3 months, it's in perfect condition and would make you a screaming deal on it.
However, both cards would need a good power supply to run them.