Consider adding a Firewire 800 SATA enclosure. Then you can externally add an SSD or an HD for increased throughput (e.g. put the root on the internal drive, your home on the external one; or using OS X logical partitioning to combine both drives into a single volume)

Dont bother with a top of the line SSD, since the FW 800 can be saturated by mid-range SSDs.

I have done it and it's not bad. Well worth the trouble and makes a 2008 feel like a new machine. There are detailed tutorials online with pics and everything you need.

do it! plus upgrade the ram and you will be happy for another 2-3 years with that machine!

I have replaced drives in these models a number of times. It is not that hard. You don't have to do a teardown or anything. You just have to remove the keyboard. There are just a lot of screws, and you have to be careful with the keyboard ribbon cable.

Make sure you are on at least 10.6.8, and don't forget to install Trim Enabler. (For 10.6.8 you need version 2.2).

http://www.groths.org/software/trimenabler/