Well, it depends on what you want it for. If it's just for
occasional system backups to CDR(w), then a USB burner will
work fine. These are limited to write speeds of 4x, and basically
any USB burner will do a reasonable job. You can currently find
them for as little as $99 after rebate. Oh, and USB drives are
limited to read rates of only 6x. Not much fun.
If you want a burner for more frequent use, then you will want
a faster burner. That means you need to get either a SCSI or
firewire drive. The one you choose will depend in part on your
system. Do you have either firewire or SCSI already installed?
SCSI external drives tend to be a bit cheaper than firewire
drives, mostly because they sell somewhat better. Firewire has
the advantage of hot swap, but if this is a permanent fixture
of your system, who cares about that?
SCSI/Firewire drives are available in speeds up to the 12 or 16x,
with 32-40-whatever x for reads. However, these drives are all
going to cost you $200++.
Kwad