Welcome to SPCR. It's doubtful that your system RAM is the bottleneck for that game. 4GB is usually enough for most games under Windows. It's easy enough to see - just run the Windows Task Manager and click on the performance tab. Start your game, run it for a couple of minutes and then alt-tab back to to the desktop. Look at the Physical Memory Usage History graph. If it's maxing out, then you need more RAM.
Here's a benchmark review for the game @ Techspot.
The game appears to be light on CPU use and mostly dependant on GPU. You don't mention your monitor resolution, but looking at the charts, your card is best at running the game at less than 1080p or by turning down the eye candy (you can probably double your fps).