Your usual options are to go with a firewire or USB enclosure. I built an external 5 bay case that connects with firwire but you can add a single drive at about $30 per case. Read the big external case thread before you do anything because you can easily buy the wrong case depending in what type of drive you want to use.
Using another method , you could add an additional PCI IDE card and add your HDs to this card, freeing up your MB IDE cables for more optical drives. This assumes you have more drive bays. The Promise card is a pretty good brand but it won't work on all optical drives so plan to use it for your HDs. You could also go to an SATA card but this is more limiting and more expensive as well and offers no additional benefits.