If you have 2 IDE channels, you can put 4 devices on it.
If not, you can use a PCI add-in card with an IDE controller
on it. They're made by Promise, Highpoint, SIIG, and several
others. I use the SIIG controller card which accommodates
up to 4 devices:
http://www.siig.com/product.asp?catid=103&pid=437
SIIG also has them for serial ATA devices:
http://www.siig.com/product.asp?catid=103&pid=467
You can use the model nos. to search Nextag.com and
Pricewatch.com for the current "street" prices.
When you get to stuffing lots of devices into the tower,
cabling can get to be a problem for both routing and
air passage. I use "round" cables for IDE parallel ATA
devices (also the floppy drive), and they have worked
well for me for 2 years, now. I prefer the kind with the
aluminum braid shielding:
http://www.svc.com/cables-ata-100-133-round-cables.html
Here the aluminum shielding is called braided "silver":
http://www.svc.com/rc18hd1.html
"Round" cables don't conform to the formal ATA specs,
which designate 80-conductor ribbon cable, but these
use 80-conductor twisted pairs to accomplish the same
thing - each data wire is twisted together with a ground
wire. "Round" cables also come in a range of lengths
and in both one- and two-device configurations.
If you go the removable tray route for your backup HDs,
you'll be amazed at the convenience and versatility.