I'm firmly in the camp that you can never have enough storage inside your DVR. With the often miniscule price difference between hard drives with significant capacity differences, I think 500 GB should be the minimum. My junk DirecTV DVR only has 30 GB of storage space. If you're thinking about buying a TiVo Premiere DVR, there are some indications that a higher capacity device may be coming soon.
There has been no official announcement from TiVo on capacity upgrade, but a model that has clearly been refreshed has been turning up at retail locations for pre-order. Listings at Amazon and JR.com are showing a new TiVo Premiere DVR model carrying part number TCD746500. That particular part number packs in a 500 GB hard drive and sells for $149. The original versions had 320 GB hard drive inside and sold for $80.
The listings imply shipments of the new 500 GB units will start in the next few weeks. If 500 GB isn't enough capacity for you, you can still grab the 1 TB premiere XL or other models. The 500 GB version should be good for an extra 30 hours of HD programming, which for many would be worth an extra $70. The hard drive size appears to be the only difference between the new model and the old.