This configuration is way beyond the minimal needs for such a system. You don't have to worry.
In respect to the MythTV part, I have made a few readings only, and I can't tell you much. But there are pages around, google for it.
There are dedicated projects on how to build linux MythTV boxes, but they don't include sharing internet connection, so you are gonna have to do research on which one is the most aproximated to your needs and easier to complete customization.
Concerning the firewall/gateway part, you should know by now that these apps do not consume much processing/memory unless you are serving for several boxes (not the case, right?). In the other hand, mythtv does consume.
A few points to consider:
- An athlon 1900 isn't the most economic cpu around. If you plan to leave the gateway powered on 24/7 you are spending a lot of energy. People are building gateways with 486's, very economic and useful.
- If you are a basic user of linux, perhaps you should consider picking a friendlier distro, such as redhat/suse/mandrake. any one of these will do the job. Actually, I managed to share an internet connection with crossover cables, just like you, with these three distros.
- The bttv based card is, to my knowledge, on of the easiest tv cards to setup under linux. Good choice. I have one of these and suse has no problem working with it.
This is a Athlon cooling and powersaving howto. Nice thing to have on an athlon box.
I should have a few more links that can help you. Let me search and I'll post them later.