Marci: the 'noisy' generation of 15K drives are prehistoric :P I remember having those 15K IBM drives in a server, noisy as hell (they sound like a jet engine), and hot (temps were over 70°C) ! But now 15K drives are quite silent, thanks to FDB, and quieter even than 10K or 7.5K drives, since the noise frequency is at the limits of human hearing bandwidth. Even the seeks are toned down (that's because of the lesser travel of the actuator, platters are 2.5" instead of 3.5"). The Seagate 15K.3 is awesome for that, only equalled by the Raptor which beats it in the price department.
TaTs, from your drive capacities (and temperatures) i conclude you have only ATA drives, those are 7500 rpm max, and nowhere near as hot as 15K SCSI drives. Actually most ATA drives don't need any active cooling, heat transfer through the sides into the PC case frame is enough. IMHO you will better go with a silent enclosure like this one:
which is excellent and quite enough for 7.5K drives (i use it for raptors, which do very fine in there). No need to blow air over them, they're designed for passive cooling. Just make sure there's room around them so convection can do its work.