If you have the money, I would get the 3500PRO RAM. Just beware of your HSF as this RAM istaller and may not fit. Do not get SLI. As most people will tell you, the price vs performance gain just isn't worth it.
The computer will run better and more stable with the new RAM and will give better numbers in benchmarks, but will you SEE an increase in performance is hard to tell aas we don't no who poorly the system is running now.
I say go for it.