who has tried over clocking the at89c51 in KEIL C? how fast can it go? 10% 20% 100%? post your results.
I rather suspect that speed will be the least of your problems:
The AT89C51 has only 4K Flash and 128 bytes RAM - you are certainly not going to be able to do all that stuff without external memory expansion!
Possibly not even with the full 64K expansions - so then you'd have to be messing about with banking. The learning curve for banking is not trivial...
You'd avoid all of that (and save money) with an appropriate processor choice!
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.