Try and find a K6-2+ 550... they are out there for under $50.00... but see if your motherboard works for that cpu... not all do. Research the cpu. The K6-2 plus is way better... but it is on par with the P3 550 I think...
But the other posts are correct... what kind of an XP can you get for $50.00, an AMD Athlon XP 1700+ at least, I think. But if you are not up for upgrading ram, power supply, case, motherboard, and lots of fans, think about the plus. I both build a completely new system and got a K6-2+ 550 cpu (it overclocks great to 600 and 616 MHz), and my friends use this old computer... there is some lag over a lan on newer games, but for a lot of older games, it is still a great setup.
<font color=red><b>To reign is worth ambition though in Hell:
Better to reign in Hell, than serve in Heav'n.</b></font color=red>
John Milton, <i>Paradise Lost</i>, II 262-263