Your self-powering requirement is going to limit you to a notebook-size drive (2.5"). The first 500GB notebook drive has just been announced, but is not available yet, but currently 250GB is the limit on what's easily available in 2.5".
I don't have a specific recommendation: what I did a few years ago was get the drive I wanted, plus a USB enclosure for it. Have a look at e.g.
Komplett for examples in the £10-20 range - the first Akasa models are IDE-only (not a problem) for £11. So you're not limited to the external drives that are available, if you're a little adventurous. e.g. a Western Digital (WD)
Passport II 250GB external drive is £99 on Komplett, while the Akasa enclosure + a WD 250GB drive is £75+11 = £86.
PS: on my 6-y.old. laptop I need to use a cable with 2x USB plugs (supplied), just to get enough juice to the drive, but that wasn't needed with a newer laptop.