First thing I'd suggest is a new mouse. I prefer the logitech mx 700
link
Other things you might want:
-speakers (you may or may not have some), if not, I prefer a simple 2.1 setup like
this Obviously not a high end setup, but for $55 you can't beat it.
-monitor (os x is sweet w/ a large monitor, may I suggest a new or old 20" cinema display?)
What else do you use your computer for, do you have any specific needs?
btw, don't buy your RAM from Apple, buy from crucial, OWC, or others