External USB Drives - 400+GB

Need some help ...

Have a dual processor, 3.2 Ghz chip machine with SCSI, two large hard drives. Between audio, podcasts, and pictures, I now have filled the 300GB disk I use for My Documents. I also have 3 open bays inside the computer.

Strongly considering a 500GB USB external drive (Maxtor or Seagate) for storing "content" -- portability; backup (eventually); ease of use; etc. Also don't want to add "extra heat" inside the case with a large hard drive.

Tradeoff seems to be access time and speed.

Is my concern about heat off base? Are there other brands I should consider? In my past research, many user comments talked about the external drives running hot and burning out sooner vs. internal drives. Is that still the case? Or is technology much better now? Should I fill up the internal bays before using external drives?

Would appreciate any help or advice. Thanks in advance ...