I think an 800MHz FSB CPU will work fine, it will just run at 533MHz FSB, so you will see a loss of performance.
My advice try and hunt down a Northwood P4 not the Prescott. I think the older Northwoods use a 533MHz FSB anyway. Plus they run cooler which is a consideration in a small form factor chasis such as yours. Northwoods are in short supply but they can still be found. Trey here click here, or click here, click here. You could always try a Celeron if you are only using office apps you will not see a difference, somthing to think about?