StinkeyFingers,
In reading the information you have provided thus far, I think I might be able to provide some more.
I also am on Comcast and have similar package as you. My box is XP Pro P4 w/ HT 3.8 Ghz, 2GB RAM, 10/100/1000 NIC. I run the TF2 server just fine and it appears on the Steam server list. Furthermore, ppl can join it w/o problem....sometimes.
Recently, it did crash as basically all 7 joiners did not have a map that was on the server, thus downloading it. It seemed to be enough to crash it. Since you are a home based machine and w/o fast DL service, I would suggest the following:
1. Disallow client uploads
2. Set your region to more appropriate (e.g., East or West, not World)
3. If you have more than just default maps, then tell your friends to obtain them separately and install, rather then D/L from your server.
4. Last thing. Here are the ports I am forwarding to my machine, which is statically assigned on the private address. My public IP has changed a bit often in the last week, but that has had no impact on my setup.
UDP 1200
UDP 27000-27014
UDP 27020
TCP 27030-27039
TCP 27015
(forgot my source on this)
edit- according to the following link, only 2 ports needed:
TF2Port
Other option -- is to DMZ the private IP address of your machine. Course I only recommend that if the machine is only being used as a server w/ no sensitive or important stuff on it.
I don't claim to be an expert, just want to share my experience as it is somewhat similar to yours.