Sunfire V120 Hard Drive Speed Question

I'm looking to drop another HD in an old Sunfire v120 - and we want to put in something bigger than the 36 gb drive that we've got. I've been scouring anyplace that can get me something fast (working under a time crunch) and so far all the drives I've come up with at 15000rpm. The specs seem to want a 10000rpm - now I know some things are flexible (like size) - but what about speed? Is it going to cause heat issues?

Help - I need to make some decisions about ordering here asap.