Superfetch helps to improve performance. As Wile E says, it may help a little bit in very memory intensive applications, but generally speaking you're not going to notice the improvements in games I doubt.
It links into the way I was telling you that Vista automatically free up ram when it's needed:
Superfetch is similar to Windows XP's prefetch, only more advanced. It will automatically load parts of the applications you use most regularly into the system memory allowing them to be launched significantly faster. Obviously this will result in Vista using quite a lot more RAM. However this has almost no impact in real applications since Superfetch will automatically unload the the memory being used if something like a game, which needs the ram, is launched. Therefore, I would quite strongly recommend against disabling it as it makes day-to-day tasks such as office use and web surfing quicker without any real impact on gaming. Feel free to try it if you wish, but for me Vista feels quite a bit slower without it.