who has tried over clocking the at89c51 in KEIL C? how fast can it go? 10% 20% 100%? post your results.
Or why not get a Raspberry Pi? http://www.raspberrypi.org/
The big issue here. Do you want to build one or many devices?
If one - get a development board with ethernet hardware and a prototype area.
If many - get a development board with ethernet hardware and a prototype area. When you are happy: make custom circuit using that CPU.