"Requirements Minimum 350W or greater system power supply"
That's really low wattage power supply...really cheap too...

You only need one free pci slot for a gfx card, what other cards are using the slots you have?

Just open your box up, check the wattage on your current power supply and see whats up with the pci slots. I mean, if you want to play halo 2 on xp, feel free, but you cant play online..