I want to maximize the speed of the connections by using more of the available RAM that we have in our server (there's 4GB worth). What files would I need to edit, what directives would I need to add, etc.? How difficult would it be to implement something like this?
Linux will automatically manage and use the available memory for caches that speed up Samba (or any other I/O operation really). What you can do is tune swappiness and how the kernel will deal with those caches and buffers. You can read this for understanding/tuning swappiness and this about buffer invalidation and times.
A good reference for other performance tunings on samba is the O'Reilly book, there's a part of it online here