I recommend the Fujitsu MHT2020AT (laptop drive)
Reasons?...
Cheap at around $60.
Quietest drive available-virtually silent. Some of us prefer to hear our music.
This is an important one: The Fujitsu has extremely low power consumption: 0.90A maximum at spinup, an thats for the 40gb dual platter version. 20gb single platter version is probably a little more than half of that.
Remember any drive pulling more than 1.7amps at spinup will eventually fry your Ultra's power supply. This means any 3.5" 5400rpm drive containing more than one platter, or any 3.5" 7200rpm drive is out of the question. Check the power requirements of any drive you are considering.
If you will be running EOS 4.7, you could go with the 40gb version of this drive. Keep in mind though, if you prefer to avoid the EOS "loud outburst" bug and as a result are running EOS 4.6 or earlier, the Ultra will only support up to 20gb, so the 20gb version is perfect.
Trust me, stay away from 3.5" drives, they are shite for use in an Ultra.
...You'll need an adapter like this too:
http://www.buy.com/prod/StarTech_com_2_ ... 44935.html