As far as how much additional space in a more detailed look at it...
The internal drive is a 300gb (HR20), 320gb (HR21), or 500gb (HR22). Approx. 100gb is always reserved for Directv usage (some debate on the exact amount, could be a bit less). So, you actually have about 200gb HR20, 220gb HR21, 400gb HR22 which brings you back to the numbers houskamp posted:
houskamp said:
aproximate increase:
500gb = 2x
750gb = 3x
1tb = 4x..
Remember that the receiver will only use one drive (either the external or the internal when the external is not plugged in). So if you bought a 300 or 320gb drive and put it in an external enclosure, you would have the same amount of recording space as before!