You don't need more RAM, it'll just be a waste of money. 8GB is more than enough for any game. The thing that's truly keeping your fps down is your gfx card, the 8400GS is literally a low-power card not meant for gaming. I would personally try to get at least the 560 TI if you're going with Nvidia, though the 550 Ti has great performance compared to the 8400 GS. Though if you want something more 550 Ti-ish, you can take a look at AMD's 6850, it's slightly costlier but a far better choice. Or the 6950 2GB, if you have the money.