I wouldnt put ram in a pci slot!!. Also there is a point at which a large ammount of ram wouldnt even be used. As a general rule, 8gb is more than enough . Some exceptions might be if you edit hd video, or are recording games while you play, and have a ton of things running in the back ground. Also some motherboards have limitations of max ammount of ram supported. Last.. if you dont have a 64 bit operating system you wont be able to use more than about 3.5gb, because thats all that 32bit windows can support.
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.