I'm currently putting together a myth box using an old machine I have. It has an athlon xp 2700+, 512 MB RAM, and a Hauppage PVR-250. I'm considering adding an HD capture card, but I have concerns about the speed of my machine. Will this be able to view HD content well, and if so, how about recording or simulaneous playback + recording?

I am/was in a similar situation, AMD 2400+ w/nVidia 6200 AGP 4X. With XVMC, processor averaged about 50% when playing HD content. That was great except I kept having what I think were XVMC playback issues possibly related to the 6200, nVidia driver, and/or audio card.

I have since gone to the "Dragon" spec. No more pauses, stutters, or crashes. Well worth the upgrade once I can get my component transcoder working.

Anyhow, I hope that helps. You *might* avoid the XVMC issues I experieced with an nVidia 5200 series card. Just don't quote me on that

Athlon XP 3200+
2 x 256 PC3200
NVidia MX440 128mb (before)
6600 GT 256mb (now)

The MX440 actually played HD better than my 6600 due to a driver and known hardware issue. With that setup there was no issues watching recorded HD at all.

You will see other issues initially that will cause stuttering and jittery video, but those will most likely be attributed to the signal quality that you are getting.

Good luck. Oh, and once you go HD you won't want to go back. I have 2 HD cards in my main Myth Box and my wife complains whenever it records anything in standard definition.