I will personally guarantee that you will find more of an increase in your game if you get a solid state drive and install it on it than you will by using 8 gigs of ram.
What if I only have 1 GB of RAM?
In any case while I'm not an expert on RAM usage in games I would think that generally more RAM helps if a game is running slowly and a computer doesn't have a large amount of RAM to begin with. Especially considering how much of the RAM could be taken up by the OS, other programs, etc.
I would love a solid state drive, though. I'll probably just wait a few years for the prices to go down first.