I think that EPIA PD6000E, which features a fanless 600 Mhz Eden chip is
still quite faster than a 266 Mhz Geode chip (which the m0n0wall is
primarily designed for), coupled with a 256 MB RAM I did not run into
any hardware problems with VPN.
It might even be packing enough power, and as you said there is always
the PCI slot for a VPN accelerator if needed.