Newegg's got a couple 160's at 5400rpm for ~$210 --- add ~$20 for a good enclosure, and you're set.
http://www.newegg.com/Product/Produc...82E16822148073
http://www.newegg.com/Product/Produc...82E16817145658
That's the enclosure I use. I think I had to shim the drive a little if I remember right, but I've had zero issues with the chipset and interface hardware (usually what kills an otherwise good case).
If you get spooked about using only one USB cable for power on a 5400rpm drive, it provides the ability to use a second USB port for power only.