16gb is the sweet spot really anything above that can be waste if not doing a lot of ram intensive work. For the processors if doing a lot of work that requires them then ~$70 for the x5650s is damn cheap way to get roughly a third more processing power, don't get too hung up on the speed though as it is a direct linear idea. So if you were to go with the x5680 at 3.33 over the 2.66 of the 50s you would get little over 25% more speed for roughly triple the price.
Your replies are much appreciated. Thanks for all your help!