If I buy a lot of ram for my new iMac (16GB), is there a way to make the system cache my most frequently used applications so the launch time is much faster?
It's an oversimplification, but this is essentially what Unix does with its memory in the first place. If you don't shut down your computer and don't close your apps, your most frequently used apps will always be in memory ready for you to use. Apps you don't use will get swapped to disk (assuming you ever run out of RAM :-)
In Lion especially, you can almost forget about quitting apps by hand.