I sort of have a system simular to yours. It's a 1.33Ghz thunderbird @ 1.4Ghz w/ 768 megs SDRAM of RAM and a Gainward Geforce 4 4400. With my system, I get 8100 3d marks.
First
What OS are you using?
second
What video drivers do you use for your card?
and
What video card did you have installed on your computer before installing your card?
Finally
Is this a clean install of the OS?, if not, then you may have some of the old drivers from an older card.