ARM Microprocessors is it suitable?

some days ago i had an offer about very exciting project about Computer LAN Networks!
but this project is not an usual project for me and i must gather some rare informations
from the web to implementing of it...
during these days i understand that i must use a microcontroller with high speed option more than 50MHz because for having a connection in a 100Base Ethernet network need 50 MHz speed for processing packets and frames to transceive... AVR microcontrollers is not suitable because they work up to 16 MHz and is very low speed for this purpose... having this problem cause me to finding ARM Microprocessors that worked up to 400MHz and is suit for a OS such as linux,WinCE,Symbian,... on some processing devices!!!
Atmel Company have some ARM processors with AT91SAM nick name and i think it is proper to use ...
ARM Micros are some series with different characteristics in ARM7 ,ARM9,ARM10,ARM11 generations.
I thinks AT91SAM7S256 is good for my project this microprocessor is in ARM7TDMI Range.
but i am searching in the web for its limitations and abilities that may cause me...