As Mytob suggests, it all depends on what you want the machine for.
P4 adnd PM are fairly high performance; celeron fine for office / internet work.
Antlon 64s are good all rounders and some suggest that they are the best. Certainly you get more for your money.
When purchasing a new PC though, there is more to take into account than simply the processor speed. Other components will have an effect on overall performance.