It's low because you are using drivers before the ATI 8.07 beta (also loosely know as the "Catalyst 4.11 beta") drivers, which fixes the 256MB bug on all AGP X800 cards, PRO and XT.
With the new drivers, you will score around 5000 (probably about 4600 with no overclocking on the X800 Pro and about 5200+ with overclocking the card). Generally speaking, people are getting about 1100 more points with the new beta's in 3dmark05 (strangely enough, 3dmark03 performance is about the same).
I went from a score of 3900 to
5000 by going from the Catalyst 4.10 drivers to the 8.07 betas. This is with my card running at 520/520.
However, I've switched back to the 4.10's simply because I've gotten very used to the Catalyst Control Center and the 8.07 betas only come with the old control panel.
So, if you want to check out what kind of performance you can expect in the future, you can either download the 8.07 betas from somewhere like
guru3d or just wait until the official Catalyst 4.11 in a few weeks, which should have the fix.
Best of luck!