Help buying first Tivo

I'm considering buying my first Tivo and I'm looking at a new Series2-DT. Is there ANY difference between the 80 hour and the 180 hour Series2-DT units, other than the hard drive size???

I'm thinking of upgrading the hard drive immediately after I buy the box (I'm a long time Linux user and it doesn't look very difficult). If the hard drive is the only difference in the 2 Series2-DT models, and I'm going to upgrade the hard drive, is there any reason for me to spend the extra money on the 180 hour unit? (i.e. does the 180 hour unit come with better hard drive cooling or something?)

I'm thinking that if there is no other difference, I should save money with a 80 hour unit since I'm going to replace the hard drive anyway. Plus there is a "web special" on Tivo's web site on the 80 hour unit for even more savings.

Also, what recording quality are the hourly ratings (80/180) based on? I'm guessing it's not the highest quality setting? Any estimate what the hourly ratings for those stock boxes at the highest quality recording level?

Any other suggestions for buying my first Tivo would also be greatly appreciated.

Thanks in advance!!!