Use Option #2 and choose your basic system (probably a Dimension?). Choose Manuals. Look up your system in the column on the right. Click View for the Service Manual, then click Technical Specifications.
It won't tell you eactly, but it should give you a range of speeds and maybe FSB speeds. From there, either post what it says, or look up the different speed CPUs on NewEgg and see for sure.
But I gotta go with Pat... odds are it is a 478. Your limiting factor for a new CPU will be the limit of the FSB of your motherboard.