This is based on your requirement. If you want the throughput is very high, you need some powerful CPU. Ask you chip supplier the CPU LAN port performance, then you can select. Embedded Linux is not the most important problem about the Speed Requirement.
Added after 2 minutes:
Some advice, Freescale(ori Moto) has the MPC8XX or MPC82XX, they are powerful. You can check the performance in the DataSheet.
Some MIPS and Intel CPU are also good at network dealing. But the most important is that almost no one can do wired speed!