You'll want to take the external drive anyway. From recommendations from touring musician friends, you want two external hard drives for the road that have the same files backed up as a just in case.

Your system will always run smoother if you are sourcing the audio files that you are playing from a hard drive other than the boot drive that the applications are sitting on. It's just the way to do it. I suggest velcro pads that you can slap things up on walls or onto other gear with. Stick with the external drive though. The chances of audio failure in play back is far decreased if you do.

It is absolutely a BAD idea to do audio or video off your boot drive. So you're going to want to have an external drive, which can certainly be a 7200 rpm drive. Absolutely you will see a performance difference between 7200 and 5400 rpm in many uses. And an external drive doesn't mean having to carry a lot of cables; use a 2.5" FireWire drive and it can power off the computer just fine. I do this all the time.

As for the extra heat, it is likely that the computer will shut itself off before any real damage occurs, but bear in mind that heat is one of the most prominent sources of electronics failures. Exposure to heat doesn't always result in immediate danger, but can certainly lead to a considerably shorter life-span for any piece of electronics.

Get a 2.5" 7200rpm FireWire drive and you won't be disappointed.

Service Source is the set of repair manuals that Apple puts together for their certified repair folks. Sometimes you can find them floating around online. If not, try iFixIt's guides:

www.ifixit.com/Guide/Mac/

i wouldn't expect the 7200 to be significantly hotter, tho it does consume a significant amount more power...

except for that windows part. :D