I had recently bought a 320GB 7200rpm HD from newegg and I'm in the process of installing it now. It's easy as long as you have very very tiny phillips head screwdriver and very very tiny allen wrench.
And remember to unscrew that one screw that's illustrated on the back of the battery/HD cover before you tug on the tab to lift the original HD out. The allen wrenches are for the 4 screws that are on the side of the original HD that are used to seat the HD in place.
I'm using the original 320 5400 HD as an external one. Along with my purchase of the 7200 HD, I got a free external USB 2.0 2.5" enclosure. Very useful.