Nvidia 7xxx Series/630a info 1080p.....Maybe I don't have any exp with that stuff(i do know a Intel 945G will play 1920x1080). it SHOULD play blu-rays since its HDCP compliant
As for the CPU usage, I wouldn't have the slightest clue
That said, recording OTA should place little to no load on the CPU. Digital ATSC broadcasts are already encoded in MPEG-2. You're basically just writing those streams to your HDD when you record. No encoding is taking place. The HVR-1250 doesn't have a hardware MPEG-2 encoder but you only need that if you're recording analog anyway.
Heck, my lowly Atom/ION build was capable of playing back Blu-ray rips while recording 4 HD streams at the same time (I've got 4x HDHomeRun's/8 tuners total shared amongst the HTPC's). It's actually the HDD that can barely keep up with the load. Alas, the HDD partition needed to be near empty and formatted with 64K sectors or I'd get snow.