HD Tivo from Directv is tough. They lost the relationship w/ Tivo and they currently don't their Directv HD DVR out yet. The HD Tivo is MPEG2 and Directv is already rolling out their MPEG4, albeit slowly. I have an HD Tivo upgraded to 550 GB of space. It's slow but it works and it's Tivo based.
If I were to start today, go w/ Comcast (or your local cable co) and use their HD DVR on a monthly contract. DirecTV requires a 2 year commitment, which sucks. In about 6 mos, Tivo will release their Series 3 Tivo which takes 2 HD cable cards has 2 built in HD OTA tuners, supports all new s/w upgrades, and supports an external SATA Harddrive. I've heard estimates that this will be sub $500, maybe less. I will drop Directv in a heartbeat for this.
