Why not just get a decent DD-WRT router and then just use this PC + Server 2008 for running your VMs?
I have the ASUS RT-N16 with the latest
BrainSlayer-V24-preSP2 build and it's been running like a champ. I do run torrents as well, but not like I can get a ton of connections with only 5Mbps/800Kbps. Don't think DD-WRT supports MLPPP (if your using single link, maybe dual-link tho), so you'd probably have to look into Tomato.
pfSense is a great router distribution based on FreeBSD. Doesn't need that kind of computer though, and it doesn't support MLPPP.