pentium 4's (esp the prscott based ones) are very OCable the limit is supposed to be 10Ghz but the record is 8.4 GHz
Heh... For some reason I have this picture in my head of a honda civic red-lining at +8k RPM in 1st gear😛
But anywho for the cheapest/solid OC I would say maybe the E21xx series. All of them are under $90 and all of them (from what I've read) can reach 3Ghz if OC properly. Not to shabby. As far as max OC go I've read the E2180 go to about 3.7Ghz, but not suitable for 24/7 operation. But a nice 24/7 OC is almost guaranteed at 3Ghz.