Generally speaking, the higher the clock rate, the faster the CPU...at least this applies to CPU's in the same family/architecture. Comparing two CPU's with different architecture based on clock rate is foolish, and typically doesn't mean anything. But, if you got a CPU from the same family (example, core2 duos, although I believe those are clocked much higher out of the box), then sorry to say, you will probably notice a performance increase, unless you only use the computer for everyday stuff, like internet, word processing, maybe watching the occasional movie, then you will very rarely, if ever, notice slower performance. Usually the only time it becomes noticeable is when the system is under load.