Yes fox5 is right, athlon XP's are obsolete, done even go there. The pentium 4's are soon to be obsolete too, if your friends short on cash he might want to wait a month for conroe to come out. Because then, even if conroe is still a tad too expensive, the older socket 775 pentium 4's will be available for extremely cheap prices, especially the dual-core varients. Single core P4's IMO are a rip-off when you look at the pentium D line. Two cores for a little bit more, two cores with speedstep and on a cooler 65nm process.
These P4's will perform the same as the 800mhz ones you refer too, judging by the athlon xp suggestion i assume you mean a northwood P4, todays prescott socket 775 varients are much the same tbh, everyone just hates it because they run hotter hotter (90nm cpu's only) and it didnt deliver any extra performance on release or ever.