This is exactly what I was talking about earlier - it is pointless to buy separate enclosures and drives for this use.Now for just an external usb drive, I'd simply get a good enclosure that's got a built in fan - like the AMS Venus series (vary from $39 to $49 depending upon the drive type you want (SATA or ATA)) and pair it up with a nice large 500gig drive from Newegg (~$110) and you've got a great 500 GB USB external drive ready to go for roughly $150.
You can get an external 500gb hard drive, already assembled, with an enclosure that is tested as working with that drive, for $120-$130 for name brands.
Not only do you save $20-$30, but you don't have to select either component, and you don't have to assemble it (and have a chance of making a mistake).