A friend plans to buy a new 5-boxing computer but has a somewhat tight budget. Will a 4 core Intel i5 be enough or is an i7 much better? And how are AMD compared to these? I'm a kind of Intel fan myself so I have no real experience of AMD CPUs.

At least 4 cores are needed right, to make sure the CPU won't be the weak link? A dual-core CPU will be to weak for acceptable 5-boxing I'm guessing?