There are several good VHDL cores that are free. Here is a link for one:

http://www.cs.ucr.edu/~dalton/i8051/

I started using VHDL about a year ago and found the examples quite interesting to look at. You also might look at some of the software cpu emulators for ideas. There is a good one for the palm pilot as well as several for the amiga and apple II computers. These are great places to how much work is involved with getting it right.

Regards,

Bobby