The drive will work fine, and it will boot - provided it's 10.5.6. However, the new machines ship with a newer build of 10.5.6, which includes the new Energy Saver panel and proper display profiles (among a host of internal changes, I'm sure).
I think the easiest way to get on the new machine is to connect your old machine in Firewire Target Disk Mode, then migrate to the new machine. Once you have everything the way you want it (you might have to reenter some serial numbers and the like), you can do a "restore" (using Disk Utility - do an "erase destination", too, to enable block copy) from the new machine (5400RPM) onto the 7200 RPM drive. To make life easier, I'd suggest getting an external case for the 7200 (or the 5400, depending on how you want to do things, as you can boot directly to the external).
The end result should be two drives with the new build (from the new machine) and your apps/data migrated.
The alternative is to manually copy your data and reinstall apps, but I've done the migration many times now, with few problems (as noted above). The only other thing I can think of that didn't come across were some USB<>Serial Adapter drivers.
btw: my info has the current public 10.5.6 build as 9G55 and the build which shipped with my 17" Unibody is 9G2141.
