The power consumption of the drives is what's important. ATA-33 5400rpm drives are what the system was designed for; the 7200s seem to draw too much juice and get too hot...
We have used Seagate Barracuda ATA II ST330630A 30gig 7200rpm drives (http://www.itreviews.com/hardware/h164d.htm) on some very heavily used machines (RAQ->RAQ2 and RAQ3i->RAQ4i). No problems with non-stop use for over a year on each.
This wasn't a 4r with Raid.. so maybe it would get too hot with two in a box. But I see alot of warnings to never use a 7200rpm at all. This particular model HD is built very tough and is fast with the 2MB data buffer. A benefit that it takes care of processes faster (and can "rest" more than a slower 5400rpm).
I've also read from a respectable source that increased heat inside a HD will not affect the actual running performance of a HD. (Although it might cause it to totally die faster or prematurely).
Brian DJ.NET