No idea whatsoever since I never upgraded the P4 northwood machine I got, but since your new one has a bigger L2 cache and 1ghz stock advantage... I'd guess a pretty noticeable gain. I know, I shouldn't give such an obvious answer, but sorry man, I have no experience with any other P4 then the one I have.
Also you don't need to reformat and reinstall XP... though its best to do so, but XP will run fine if you don't change out the mobo, but even if you did, XP would still run.
C2D E6600 @ 3.15ghz w/ 1.35 vcore
2GB G.Skill DDR2-800 (4-4-4-12)
XFX 7600GT xXx