I don't know how much money you are lacking, but it is entirely possible to upgrade your system to a Duron 600 for less than $200. Prices may be higher where you are, but they shouldn't be that much higher.
If you just want a faster computer, you could just buy the cheapest Socket A motherboard you can find and the slowest/cheapest Duron chip. If you want to overclock, you'd have to get either an Asus A7V, Abit KT7, MSI K7TPro2, Epox something. But if money is really tight you could just go economy all the way...
Name brand comps are general not upgrade friendly at all.