I'm using version 22 of Knoppmyth. I have a Sempron 2400+, 160gig IDE drive, Nvidia 5200 video and 2 Haupagge 250 tuners. It should run on that setup. The hard drive won't give you much space to record anything. Knoppmyth uses a percentage of the drive for a live tv cache as well.
If you get a tuner that has encoding built on, it will take some of the load of your cpu. The Hauppage cards and remotes are pretty well supported in Knoppmyth. Getting the remote buttons working can be a bit of work. Once you get the hang of it though, it's very easy to totally customize your remote.
I've been running Knoppmyth for about 8 months. I think it works great. I have it mounted to my fileserver so all my videos and music play through it. I even play Sonic the Hedgehog on it.
Check out their forums and wiki for more info.
www.mysettopbox.tv
You may need to register for the forums at the moment. They are switching providers to get more bandwidth so it should be open again soon.