It's not the processor you need to worry about. Does your computer have PCI-Express, or is this an AGP version of the card? Is your power supply powerful enough (I'd say at least 400 watts and 30 amps total on the 12v rail(s)). On top of that, would the upgrade even be worth while? How much RAM does the system have? A P4 2.66GHz is pretty dated...
Your PSU is all I'd really worry about. Aside from that, that card should work fine. But once again, that processor is rather dated. While GPU's still play a huge role in game performance, you may still run into some issues with such an old processor.