Couple of tips
If you can then get 5400 RPM ATA Drive as 7200 can get a bit noisy and hot.
If you cannot get 5400 get a drive that has a utility to slow the drive down (quiet mode) - you will need to configure the drive in a PC.
If you can look at the Drive warranties. A few years back I got a Samsung Spinpoint 120Gb as all the other manufacturers had dropped back to a 1 year warranty and the Samsung had a 3 year. Drives do fail and as Tivo runs them constantly expect them to fail sooner.
Although you CAN get a huge drive think about failure. If you can archive off over a network do so onto a PC with a SATA drive or DVD - less prone to terminal failure. Also because of failure think about cost benefit of getting a smaller drive rather than bigger if you do not need the space - if it dies after 2 years it will not cost so much to replace a 200Gb than a 300GB.
I did the math with SATA drives (got a Seagate 250GB) for my PC a couple of months ago - I too have Techpac access but found they cost more for the same drives than Super Cheap PC in Auckland. So if you are in Auckland check out price spy as you will often find Techpac more costly even at Trade once you add GST and $3.75 courier. Have pity on us IT sales types we do not actually make much Margin :( and the likes of Super Cheap bring them in direct from Singapore - you run the risk of warranty based in Singapore if the retailer goes under but do the math....
Lastly there was a comment of large drive size - what is the current max size without cahce card - I though the Tivo maxed out at around 200GB?
Chris