It depends.
how much hard drive space do you have now, do you find yourself running out of space often?
How big is your current display now, do you have a dual display setup, is that what you'll be shooting for?
Do you play the xbox360, will that stuff get played?
Have you considered a faster socket 754 CPU?
What do you use your PC for?
If it was "me" I would sell the xbox and use all the remaining funds to upgrade the PC to either a Conroe or an AMD X2 with a decent PCI Express card.
If you find yourself short on Drive space, get the drives.
If you find yourself short on screen real estate, get the monitor
If you have plenty of drive space and plenty of screen real estate, and if you actually plag the Xbox360, then get it.