After the OS upgrade (10.7.5) the system begun requiring more memory (for no real reason actually) so I decided to buy more RAM to speed up the browser, email client, skype conversations and my dev VMs. The improvement allow me to keep my VMs on and talk on skype while browsing.
Smooootly. I took less than 10 minute. Nothing exploded.
Do it! Worth it definitely, though the biggest improvement will be updating the HD with an SSD.