Everything that can be invented has been invented - US patent office 1899
Since Vista brings upgrade time around again, we wanted to see what your best bet might be, if you have an AMD-based machine. To our surprise we managed to find plenty of 939 CPUs in stock. The cheapest one - AMD Athlon 64 3200+ 0.09µ SSE3 Sockel-939 tray, 2.00GHz, 512kB Cache - costs only 45.00.
The cheapest AM2 CPU will end up some 10 more expensive. You can buy it for 55.00 but you don't have much choice if you have a socket AM2 motherboard. With a cheap board such as Asrock's, you can build a system with a graphics in the form of an Nforce 6100 for just over 100.
You can check it here for Asrock, here for a 939 CPU and here for socket AM2.
It cannot get much cheaper than that. µ