miabbate
My sentiments are very similar to Matt's, though my hard drive experiences have tended to be with either Seagate or Hitachi products. Generally I would also agree that another Toshiba drive is to be avoided.
For cache, it is always better to go for more - minimum of 8MB. As a very broad indication of what to purchase, something along the lines of 8MB, 5400RPM, 80-120GB.
Also be aware that not all drives are the same, even if their general specifications are the same. For example, Hitachi produce different versions of their 7200 RPM 100GB Travelstar - not only in Ultra ATA and Serial ATA versions, but also a version designed for extended power-on usage. Drives from different manufacturers may also have varying technologies built into them. Something else to consider is the length of warranty. Some manufacturers offer longer terms than others.
15" 1.25GHz/12" 1GHz PBs, 2xPPC Mac minis, 12" iBook G4, Mac OS X (10.4.6) Cube, 2xTAMs, iPods 2G/4G, iPs, AEBS, AX