well just look in your bios and it should tell you, or call dell. but im pretty sure you got an 8x agp, but yea upgrading to a x800pro will kick a$$ because those cards move a good amount of bandwidth
O my bad man its a Dell Dim 4400 and i went to bios but i didnt find the motherboard. And the slot for the vid card is like u know all those enthernet card and sound etc. well those slots r like im the same space but the vid card slot is moved back and i dont know if ithe x800 can fit.
An x800 would work in it, provided you could up the juice (which means a new DELL power supply... proprietary parts, ya know), but it couldn't work at full speed.
You'd be better off getting an older card, either one that is 4x, or an 8x that doesn't cost so much. You'll be wasting your money otherwise.