5900 and 5900XT are not the same thing. The XT version is a little slower. here
is a good chart for GPU specs, it'll show you the difference in core speeds, pipelines, memory speed, etc, and the more of all that the better.
I would advise against purchasing an FX card at this time though, the affordable and far more powerful 6600 series will be available soon, or if nothing else I'd sooner recommend a 6800NU.