I own a HP XW4400 workstation. The motherboard appears to have 2 PCIe slots.
I own a ASUS EAH4870 1gb with A Darknight cooler. I was wonderign if I could buy a 4870X2, trifire them, and have it run at 16x/8x/8x, not 16x/8x/4x or somthing like that. I belive the motherboard may have been a 975X varient.
Then you might need a new Mobo if you want to upgrade.
Edit: 5870 uses less power at idle than a 4870, but uses 40w MORE power at load, so with a 450W psu, you might be pushing it. I'd want to get a little bit of a bigger PSU just to be safe, but you could probably get by.
I could get a 4870 Trifire, for 250$, or a 5870 for 400$, I'm not rewaly sure what's better.
Also, I could print some P4 vs C2D benches, and the Intell page where they talk about how C2D is the newest most awesome thing ever. The other month, some guy paid me 180$ for a P4 3.2ghz. I was stunned. I do some work there, I go to my local dump, collect all the P4's, bring them there, and people pay me good money for trash. I think i might try selling my rig there.