What P4 do you have? That board acording to google will take LGA775 socket for latest Intel Core 2 Quad/ Core 2 Duo / Pentium (Dual Core inside) / Pentium D / Pentium 4 / Celeron D processor.
Your best bet is a Core 2 Quad in the fastest speed you can find, good luck with that as not to many stores have lga775 cpus laying around anymore, your best bet is to find one on ebay.