Assuming you mean "record one program and watch a different program off the air at the same time", you'd need two tuners. That would be two of any of the three standard PVR cards (PVR-250, PVR-150 or PVR-350), or one PVR-500 (since the PVR-500 has two tuners built in). This also is the requirement if you wish to record two separate programs at the same time.
If you, instead, mean "record one program and watch a different program you had recorded earlier" you'd only need one tuner, and of the cards above will do.
Note that PVR-250/350 support is 'better' in the current knoppmyth release than PVR-150/500 support. The latter definitely requires updating your ivtv drivers manually.
[Ralph did a good writeup of what's required for PVR-500 configuration here:
http://www.bitbenderforums.com/~ralpha6 ... mythtv.htm
(though I think the sidetracks on the SATA configuration could be clarified a bit...

)
]
Some of the PVR-cards have a built in mpeg *decoder* chip with TV-out capability, which allows you to playback recordings w/o using much CPU as well. It looks like you were concerned about that as well, but the phrasing of the original post seemed to muddle the two questions together.
-brendan