Well i have a maxtor external case, i bought it a few years but keep changing the disk in it, and it is a very good unit.
1) How many sata ports does your motherboard have, you may want to consider e-sata as opposed to USB
2) I have never seen enclosures that stack or hold more than one drive, unless you go to a NAS server, but these are expensive.
3) Most enclosures i have seen do not have built in fans, and their enclosures are all plastic, so they give no heat off and they can get very got to touch, which is not a good idea for a hard drive that is presumably going to run very often, so external are going to have worse heat issues than you internal ones.
4) I know this is probably going to sound un-fun, but if you are going to buy external drives, the enclosures themselves are going to cost around $50 or add $50 to the cost of a unit that has a hard drive in it.
It may be cheaper just to go and buy a basic ATX case, that will give you internal slots for your HD so you can run them at full speed, and without all the messy power plug-paks that are required to run external 3.5" hard drives. You could pick up a good case for under $100 that most likley will have a 120mm fan at the front to cool your hard drives.
It depends on what you want, and how much you want to spend. For 1-2 drives, external may be just an easy add on. Considering that your existing case gets too hot with the existing setup, it might be a good idea just to get a nice cheapo case with heaps of HD slots.
Hope this helps.
PS. I have an antec 900, which you can pick up for $175, as a server it has 6 HD slots that are in removable HD cages that are cooled by 2x 120mm fans, so it would work great. You would not have the messy plug-packs, and all hard drives would be kept cool, not to mention it looks awsome.
