The SSD upgrade really is quite easy - I have limited technical skills and managed to do it in about 15 minutes.
I'm running the 2.4GHZ version of the late 2008 aluminium Macbook with 8GB of RAM and a 240GB SSD - you really wouldn't believe the difference the SSD will make. Apps open as quickly as they do on the display Macbook Airs in my local Apple store.
You can pick up a Crucial drive for less than £100 (UK) from Amazon, so I really would consider it if you're hoping to get another year or so out of your notebook.
Edit: Here you are, this is the drive I added, and it's currently less than £80. You can pick up a 120GB version for £49.99.
http://www.amazon.co.uk/Crucial-CT2...8&qid=1411398534&sr=8-1&keywords=crucial+m500