When buying a CPU,first decide what you're gonna be using it for, if you want to game, go for a Dual Core, if-not just go for a single core.
Check for the amount of L2 Cache as well, you generally want 1mb+
Never compare clock-speeds between different processor families or manufacturers, as different chips employ different technologies. Example: an AMD running at 2.8Ghz is significantly faster than a Pentium 4 running at 2.8ghz.
Also, decide whether you want to Overclock the processor or not, generally you'll only really want to do this if you're gaming, but if you do want to i'd go for the AMD opteron series, these processors can be overclocked to attain performance equal to processors in price ranges waaaayyy above it's own.