Processor for router running linux

Hi.

I'm trying to put together a board that needs to have at least 3 ethernet interfaces, 10 Mbit/s each.
It will run Linux with vpn, ssh and web server.
All outgoing traffic will be routed through a VPN tunnel. I will have at most 10 users per box generating a total traffic up to 10 Mb/s. This will probably mean that the box will have to handle quite a few QoS packets.

The box is required to have at most a 10 Mb/s throughput and a response time of at most 10 ms at all times.

I'm looking for a good power-horse for this box.
The question is should I go with an ARM based or MIPS based processor.

The MIPS based processors seem to be more suited for this task, but ARM based processors like Intel's XScale is still being considered.

Since I do not have any experience in this field, I ask your opinion.
If I go with MIPS, will a 24KEc at 480 Mhz be enough or do I need something more powerful.

Thanks for reading.

You might want to look at the alix boards they make nice routers and have multiple ethernet options for number of interfaces. The cpu varies but is typically a geode (x86 compatible). I'm currently using one as a router and have multiple vpns, several vlans, dns, dhcp server, squid proxy server and a voip server running on it and it, so far, hasn't had any problems keeping up. (Mine is a 500mhz geode with 256MB of ram of which 100MB is used as a ramdisk for the squid proxy)