I just purchased my HD set and am looking to turn my existing PC into an HTPC. Mainly I am looking to gain PVR functionality. Here are the specs of my machine:

P4 2.8GHz HT
1.5 GB RAM
640GB HDD (RAID 1)
Nvidia GeForce 5600

I would like to be able to watch one show while recording another. I would also like to be able to use a remote for tuning etc. Is this do-able with the current HD PCI cards like the HD5500?

Does this setup sound feasible?

Yes, it's quite feasible. Recording HD is not CPU intensive, so if you buy two tuners, you should be able to record two shows while watching another one (or either of the two that are still recording).
Somebody posted test files here a month or so ago, and with those, you should be able to see if your system is adequate for playback, before spending money on the tuners.