Wow.
All hard disk drives use platters. Each platter is the same size, and each has a dedicated head for read/write. When you want to make a HDD more spacious for any given recording density you add a platter.
For example, Western Digital makes a 40GB, a 80GB, and a 120GB HDD. Now, I haven't cracked the case on any of these, but on those HDD cases I DID crack when I was into this shit, what you would find would be one platter in the 40, two in the 80, and three in the 120... each with their own read/write head.
So adding more platters doesn't create more real estate to navigate.
The three factors that most impact how fast you can get the data off or on is; rpm (there are currently three spindle speeds, 5,400, 7,200, and 10,000), with the 10K being the most expensive and 7,200 being considered more than good enough; the interface (once again there are three, SCSI, Ultra-ATA, and Serial-ATA or SATA), with the SCSI being the most expensive and either of the other two being considered more than good enough. The future most likely belongs to SATA. The third factor is the 'seek speed', or how fast the head can get to the data. I haven't seen too many HDD manufacturers compete on seek speed, and this crap is already too confusing, so ignore it.
So, anyway. Whatever HDD you get it's good where your criteria is concerned.
However, the smaller the HDD, the easier it is to back up since you just capture the whole disk without picking and choosing data. So THAT may be a consideration.