As long as the Pentium 4 processor (or any other Intel processor) has an EM64T instruction set shown in the Intel table in "The Mother of All CPU Charts 2005/2006" it is 64-bit, because that is the instruction set that adds 64-bit capability.
my Athlon 64 3000+ will perform about the same in games as a 3.2Ghz P-4. My CPU costs half that tho and mine will run cooler. 64 bit or 32. doesn't REALLY matter right now. It will when Vista comes out tho.