Just wait!
A Core 2 Duo is still an very capable machine.
All the i5 and i3's are is essentially a slightly more power freindly Core 2 Duo with Hyperthreading. Even though the Core 2 Duo has been out for a while now, it's tried and tested, and still comes in many notebooks for sale today (the 13" MacBook & Pro for instance)
You won't see a difference if you do anything short of professional movie editing or something high-processor use like that.
If I were in your position, I would wait AT LEAST 2 more refresh cycles.
You have a great machine, and would probably still be able to run all the latest software for the next 4-6 years.
My 2-Cents.