This may help you with your decision:
It's the CPU chart from Tom's Hardware, and it includes all the processors you have mentioned as well as a varity of benchmarks.
Finding Primecoin blocks has more to do with luck than processing speed. I've mined 24/7 for a few weeks now with 3 different processors, and the number of blocks I've found on each system has no relation to their speed. Unfortunately, the chart I've linked to doesn't have a "CPU Luckyness" benchmark!
I'm assuming that Primecoin mining is not the only thing you are going to use the computer for, so I'd recommend looking at the benchmarks for whatever it's other uses will be.