I was also wondering, how much power is needed to make a back end Myth Box? I have a athlon XP 1Ghz with 512 MB of Ram. I was thinking I could either add more ram or a video card with TV-out and a PVR-500 and make a box where I could watch live TV and record at the same time?

Yes. See the hardware pages on the wiki - http://www.knoppmythwiki.org/index.php?page=PickingComponents

I am currently running a 64 MEG Nvidia Geforce 3 TI500 Card with DVI, VGA, and S-Video out. Will this take any load off the CPU when watching video so I can use a PVR-500 and record and watch Live TV or should I buy a 350?

I have either a 1.2 or 1.4 GHz Athlon (can't remember which, probably the former) that I use as a front/back end with no CPU issues. In fact it handles HDTV just fine too (have to use XVMC)

So no worries for a back end.

768MB
Nvidia 5200
PVR-150MCE
pcHDTV HD-3000

The intent was that this was enough CPU power even if you were going to do all the encoding in software. Further down the page it talks about how the types of capture cards and how they affect you CPU needs. Since the PVR cards do MPEG encoding in hardware, and thus substantially reduce the CPU load for recording, you should be just fine with that CPU.

My Athlon XP 2500+ (which really only runs at 1.8Ghz) typically used less than 15% of the CPU to both record and playback live TV with a PVR-250.

I appreciate all the replies. I think I am going to buy me a tuner card, then try it out. If it cannot do what I want it to, then I will just have to upgrade.