ESXi 5.0 accepts many types of Intel platforms. If you are building a computer from scratch, any i5 or i7 will be more than enough. RAM is very important, more than the processor. Depending on how many virtual environments you plan to have, you will need more RAM. At least 16GB will be recommendable. You dont need right now a RAID or any facy. A 1 TB hard drive will be more than enough.
I am creating a tutorial on how to build a PC and run several virtual environments in my blog: http://changk.net/?p=107