Instead of manually creating a fusion setup I'd suggest buying a "pre-made" fusion drive in the form of Seagate's "Solid State Hard Drive" lineup (formerly Momentus XT). I recently upgraded from a first-generation 500 GB Momentus XT to a 500 GB Samsung 840 and was pretty surprised (and disappointed) to find that for average, day-to-day use, the SSD really doesn't offer that much more over the Momentus XT.
The most impressive speed improvement is apparent when (re)booting: the time spent at the Apple screen is minimal with the SSD, whereas it still took a while with the Momentus XT. I don't reboot very often, though. The SSD will also be superior when it comes to reading and writing large files, thanks to its high maximum transfer speed. That doesn't matter much when opening things like a large email database though.
A SSD is still the fastest you can get, but in my experience and personal opinion I'm not convinced that it's worth the price premium. The Samsung 840 doesn't benchmark poorly at all, but it's also not the fastest SSD around. If you go with a SSD then perhaps consider a higher-end one. Otherwise, the best value for your money (speed and storage capacity) seems to be the SSHD line. Seagate now seems to have hybrid drives in 5400 RPM; make sure to get one in 7200 RPM if you go that route.