How do you judge what sort of CPU and memory requirements are required for
a new m0n0wall installation?

On the WAN, 5Mb downstream 1Mb upstream, I'm expecting to have 1 or 2
IPSec tunnels, and 4-5 PPTP tunnels active at any time.

There will be a LAN port using NAT, and a OPT1 which is a filtering bridge
to WAN.

LAN and OPT1 will be 100Mb or possible gigabit ethernet, and will have a
sizeable amount of traffic passing back and forth.

I'm planning to use a PC, but I'm not really certain how fast a machine
would be required.  I'll be repurposing old hardware.

-- 
Sorry, she meant to say "stripped naked and thrown out an airlock"
I'm sorry for any confusion this may have caused.
 -- President Sheridan, B5