Windows 7 will try to keep your idle (with no extra applications loaded) RAM usage around 40%, roughly. As you upgrade your RAM, as the amount it has to load is somewhat finite, this falls to 25-30%.
It does not fall as sharp as you would expect because as Windows 7 memory manager ''realizes'' it has more room, it will scale down page file usage, irrespectively of how much space you have reserved for it. I have 6054 MB reserved (equal to the amount of my available physical RAM) yet it uses 100-300 MB, probably to page out that small amount of memory that almost never gets called into use.
Superfetch (cached memory) really begins to shine when you have a lot of RAM. If you analyze it with some tools, you realize it's mostly made up of memory-backed files.
What are those memory backed files? Superfetch literally loads files from your disk into RAM - executables, dll's, Firefox, IE and Chrome cache files, and so on. As you equip your system with more RAM, the more it loads - and after it has had some idle time after boot to load'em, most applications you use often will start quickly because they don't need to be read from the disk, they are already in memory.
Of course, if you equip your machine with an SSD disk, the last two paragraphs I wrote become irrelevant.