P4 processor @ 3GHZ, Intel MB, 2GB DDR2 RAM, 80 GB SATA HD
This will be fine (more than fine, actually) .. I had no issues running a ~10mbit (symmetric) node on an old P3/1ghz with 1gb RAM (it was FreeBSD).
all behind a Linksys Firewall Router.
This will be a problem. Cheap-o routers don't have enough memory to manage huge state tables. You'd be better off getting a second NIC card for the PC and just using the server to firewall/NAT your LAN, in addition to running TOR. If that scares you, just re-use an old PC and run Smoothwall on it (or any of the other many "appliance" distros that do this).
My service provider will most likely be Comcast cable broadband.
YMMV, but Comcrap will axe you if they know you're running servers, and they WILL know that if you decide to run an exit, because they'll get lots of complaints about it. I lost count of the number of complaints mine generated, but I still have copies of the various subpoenas I got (*).
Good luck in any case! Cheers, Michael Holstein CISSP GCIA Cleveland State Unviersity(*): ultimately, it wasn't all the legal problems that made me take down our node, it was the fact that I couldn't stop folks who were stealing journal articles from various academic publishers that (stupidly) rely on CIDR subnetting to authenticate a campus.