I'm not trying to drive people away here, but here's an upgrade that I have REALLY enjoyed:
I don't use my optical drive all that often, though I still have use of it via a bus-powered firewire enclosure. Having two internal HDs on my aging machine has added YEARS of usefulness to my old machine. I've got a gargantuan Aperture library, large music library, tons of Logic files, and plenty of elbow room on my second drive, leaving my faster original drive clear enough for large scratch disk space operations in Photoshop and rendering programs. People who do a lot of video editing could find a lot of usefulness in this internal storage too.
iFixit has better tear-down instructions than MCE ever gave, but the BTO sled for you second HD is very useful. A second HD is something I think a lot people who visit this site could really appreciate.