This is part of the reason (besides allowing for expansion) for getting a fairly large case and using 1 or 2 extra fans. Constant improvements in areal density of hard drive platters has kept the number of disks in a drive down - if I remember correctly, the 80 gig Maxtors only have 3 platters. The reason this is important is that heat generation comes from the combination of total platter surface area and rotational speed, caused by air/platter friction, with bearing friction adding somewhat to the equation. Using 1 or 2 of the newer 80 gb drives in a system should not cause a heat problem. The machine I am saving for will have 8 of the 80 GB drives - That definitely WILL require extra cooling and careful planning, but since the machine will be out of the control room, noise isn't much of a consideration so extra fans and spacing should do the trick.

Heat considerations need to be looked at as a total system thing - The A.N.U.S. machines, if not pushed beyond the numbers discussed in those threads, won't have serious heat problems due to processor cooling since the Northwood chips seem to run so cool that the stock cooling supports tha amount of over-clocking being discussed here (1.6 clocked to 2.1, 2.2 clocked to 3.04).

Drive heat, if limited to two drives, shouldn't be excessive either. Mount the drives with as much air space between them as possible - skip a space if you can. If more drives are used, it would be a good idea to get some type of drive cooler, such as the front-mounted dual fan type.
The first DAW I ever built is still going strong after 5 years and has 2 5400 rpm drives, 3 7200 rpm drives, 2 cd-rom's and a burner in a 26 inch tall tower. Initially there were heat problems with drives (measured BEFORE failure instead of waiting for death) until I modified the drive cage to allow setting the drives back about 1 inch and separating them by about 1/2 inch. At the time, cooling fans for Pentiums were only about 1.5 inches, so I bought two of them, threw away the heat sinks, and mounted the fans in a home built bracket in front of the drives, in such a way so that each fan blew air between a pair of drives. This dropped the temperature around the drives by about 25 degrees F, and no more heat problems.

Overall, air flow through the case should be maintained so heat will be swept away and out of the case. This is helped a lot by dressing cables out of the way of air flow, which is normally in at bottom front, out at top rear. The flat ribbon cables normally used in computers suck for getting in the way - spend a few extra $ for round ATA cables, don't leave the other ribbon cables in a rats nest blocking air (flatten them against sheet metal surfaces, tie them back, etc)

Don't put exotic "gamer" video cards in a DAW - they take too much from other resources and generate more heat. Best deal for DAW video is the Matrox G450 or G550 dual head card. Whether you use the second output now or later, it will be there when you want it. (And you will want it.)

Generally, if you add a fan at the input side you should match it at the output - the idea is to have as much air being pulled out as is being pushed in.

I know you only asked about drive heat, but as I said it's a total package deal. Don't think of it as rambling, think of it as more for your money... Steve