I used to have a cheap machine with 4GB of RAM and an HDD. When I got some money, I went for an SSD instead of buying more RAM. The difference was significant.

I used to have a cheap machine with 4GB of RAM and an HDD. When I got some money, I went for an SSD instead of buying more RAM. The difference was significant. I happily worked on that laptop (with 4GB) for another 6 months (thanks to the SSD for making it bearable) before upgrading RAM. When in doubt, buy more SSD, not RAM :)