A decent PC should be able to cope with 4 streams. My AMD X2 4400+ can do it from 2 PCI and and a dual USB tuner.

The issue as stated by Glenn is getting the data to disk. Using MythTV I have storage groups which essentially group together disks ala JBOD or RAID0. However it then records a show per disk essentially which eliminates the problem.

Recording 2 streams and playing back one should be fairly easy with a standard 3.5 drive.

It's not the raw data rate. HD TV is only around the 2 megabyte/sec rate. It's related to seek times since it has to seek to multiple different spaces whilst accessing multiple files.

It can easily be done in the $1000-1500 range BUT will possibly require a bit of manual tinkering.

Many motherboards come with optical/coax SP/DIF output and some are even coming with HDMI these days. HDMI Audio support can be a bit tricky to sort out.

It's really as simple as a normal PC with some kind of TV output + tuners (digital or analogue). There are many variations of getting the TV signal out of the air/space/cable.

MythTV is powerful but tricky for new users and sometimes drivers for hardware can be a pain if the hardware is brand new.

MCE can also be a bit funky to set up but is generally a bit easier to use.