Help me make my 2008 iMac fast again...

I'm getting really fed up of my early-2008 iMac.

I'm running 10.8, but it is ridiculously slow, and it crashes quite often and I have to restart several times a day. I've already replaced the HDD once before.

Safari is particularly awful, and I constantly have to "reload" the tabs because "pages aren't responding". It's nightmare stuff.

This is what I want to do:

1. Upgrade 2GB RAM to 4GB (yeah, I know 2GB is pathetic)

2. Replace 320GB HDD (which has already been replaced before) with a 256GB SSD.

Yet, I've been told that this won't make a huge difference because the machine is on "its last legs anyway". Is that true?

And what the hell is causing it to be so slow? What can I do software wise to fix this up?