Disregard, didn't see GI_JONES post, which covers it all
Extra ram doesn't hurt, but it doesn't necessarily help either. A typical gamer would never need more than 8 gigs; however Windows 7 64bit will recognize up to 16 gigs. Given the price of ram, an extra 40$ for 16 gigs instead of 8 isn't a huge deal if you are not sacrificing from other parts of your system to keep your budget.
If you don't have a 64 bit OS, you can't use more than 4gigs.