Nforce4 is only for Athlon64 (socket 754 & 939), not Athlon XP (socket A). As far as I know there are no socket A motherboards available that supports PCI Express.
Anyway, I wouldn't invest too much in it, given that you'll have to dump the mainboard if you decide to upgrade to a faster CPU. Asrock makes excellent budget boards - try taking a look at K7S41.