Yes, MPEG4 would almost definitely be better. It's more CPU intensive to encode and decode than MPEG1 (or 2), but that's probably not your limiting factor here.
I seriously doubt you're going to be able to stream the video. Encoded NTSC is probably going to end up being about 2Mbps, which probably isn't going to stream very well. You could transcode it to smaller resolutions, but I don't think that's going to happen in realtime with the amount of CPU you're going to have available, so you won't be able to stream that, either.
I think your best bet is to have MythTV record the shows, then transcode them to MPEG, then download them. Even better, it might make sense to transcode them to DVD-compliant MPEG2s do that you can just burn them to DVD and watch them on your TV.
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Bitt Faulk