External Hard drive questions

I am thinking of buying ATV flash primarily because my old ATV is just about out of space! All the features of ATV flash look great but my main driver is to extend the life of my ATV without having to buy a new one with more built in storage. That being said I am thinking of buying an external hard drive and making it the primary drive, with that in mind I have a few questions

  1. Does the external hard drive need to be one with its own power source or will it run OK just via USB from ATV
  2. Is there a min RPM for good performance, I don’t want it to be slower than the existing internal hard drive and I want a quiet drive too.
  3. I would like a small (in terms of size, not storage) hard disk (320-500gb should be enough) - anyone recommend a good one?
    Many thanks

Search and you will find: http://forum.atvflash.com/viewtopic.php?f=6&t=2767&p=11655&hilit=usb#p11655

Well, the 2.5 inch drives are usually 5400rpm.

These should work, especially if you add a powered USB hub:

thanks for your help