If price is not an issue I recommend the new 7k120 from Hitachi.
The 7200 does not really use more power than the 5400, at least not that you will notice unless you notice 5 minutes on a 4 hour battery. It does use alot more power to spin up, but after that the power ratings are nearly equal.
The same can be said about the heat. Theoretically and by spec their is a slightly higher number, but I dare you to notice it with two notebooks, one with the 5400 and one with the 7200.
For value the 60GB 5400 seems to be it as it can be had for $80 on ebay or even as low as $60 shipped on rare occasion for the patient ebayer.
A hard drive upgrade is the most satisfying, especially from going from as slow 4200 to a much larger 5400 with the higher platter densities, it really quickens things up. Also the fresh install to get rid of all the clutter accumalated really spifs things up. Last of course is all that extra room, shoot now you can start storing, encoding, and watching videos.
A HD upgrade from a small 4200 to a larger 5400 or better 7200 is the best notebook upgrade for the majority of folks though if you have specific needs like fast wireless then that maybe different.