Unless you find stuff just "left on the shelf" you may have a hard time finding IDE or EIDE hard drives. They've pretty much disappeared in my local shops.
There seem to be some advantages for SATA but I haven't been forced to switch over on my own computer, and Lin's (the newer one, naturally) came with an SATA HD that she's only got about 10% filled.
I'm at about 80% on an internal 160 GB and 45% on a 180 GB external that I use regularly, so backups (for all four machines) rotate between a couple of external 250 GB drives that only get connected during backups.
The big problem with having lots of storage is that a full AV scan takes about 7 hours on my machine, and if I really lose something and have to do a full search on one of my two main drives it's about a half hour just to find out I misspelled the word I was looking for.