wkeahey, the 501 has only one tuner, so it does not record two programs at once. (The unit that does is the expensive 721.) I'm not sure of the size of the hard drive but Dish advertises it to hold up to 35 hours. I actually got a little more than that when most of what I'd recorded were old black-and-white series. The 501 records the actual satellite stream (no degradation), so its capacity depends on the compression Dish uses for a particular channel or program.
As I'm sure you probably know, the SA TiVos will work fine with any current Dish Network receiver.
As far as connecting to a phone line, Dish Network's agreement says you must have the receiver connected. In practice, however, it's not necessary.