Celerons are crippled Pentiums... I've heard the likes of "To work out how a Celeron will compare to a real Pentium you half its clockspeed" thrown around, but they're architeture and everything else is built down to a price, so the two can never really be compared on equal terms...
Dont think you could get away with doing what you suggest TBH. I know if you add slower ram to a system, then the overall FSB will be reduced/bottlenecked by the ram, but I would imagine trying to force a CPU with double the Mainboards supported FSB and cache onto the thing would be asking for mucho trouble! Then again, Is it even possible, as generally the mainboards with the required socket, also have the required spec to support that CPU range, if that makes sense.