ReplayTV 50x0, where x is a 4, 6 or 8. 5060's are reccomended because I beleive all of them are actually 5080's if you're willing to open it up and "patch" the drive. I've got a 5040, and never run out of space, but I watch TV most nights so I don't get behind too often. You'll want one with a lifetime subscription. Last time I checked pricing was still going up on the 5000's, so it's a decent "investment".

Matthew