according to the gpu database at TechPowerup click here a 9800xt should be 412 core and 365 memory. (just a bit faster than your 9600pro @ 400/300, but with much greater bandwidth)
The only ATI gpu's in the list with 450/250 timings are X1300 cards which do come in AGP and PCI-e versions.
Run Belarc advisor or Everest and see what it identifies it as. Or even the Win XP system info.
Google for some images of 9800xt and x1300 cards and see if yours looks like either.