There are lots of threads on this subject already. Just do a search if you want to find out more.
If you got the money, then I would get a pentium 4 processor. If not, an AMD will run just as good but for alot less. I've heard that AMD runs at much higher temps than pentiums, so its advisable to get good cooling. I think Pentiums are better than AMDs if you want to do processor intensive tasks such as video editing/burning/ etc.
What you should get, really depends on what you want to do with the system.