All games are written for the 32 bit environment to maximize the marketplace. I know of no games that are written exclusively for the 64 bit environment, and none that have a 64 bit enhaancement for 32 bit games. Originally, 32 bits was a massive amount of ram. The os got half, and the user area got the other 2gb. Since the os does not really need more than 1gb, there are some mods to allow a game to use 3gb.
The value of 8gb or more comes from minimizing the interference from other tasks.
Here is a corsair study on the value of >4gb:http://blog.corsair.com/?p=65
The other way you could use lots of ram would be to create a ramdisk, a superfast, small hard drive image.