get an amd athlon xp 2400+ which performs at 2.4ghz p4 speeds at around 70 dollars, though you need a diff mobo, try a cheaper brand such as ASROCK (socket A)
Yeah, you can go with the Athlon XP series too. Since AMD is cutting the Athlon XP line, they will be really cheap (not to mention quite a collector's item, after a while ).