You would be better off using 2x2GB sticks (you can buy them in a matched pair) giving you 4GB RAM that you can use in dual-channel mode. This would probably give better performance than 5GB of mixed stuff.
There will be a very noticeable difference when gaming with your new CPU due to the extra core that the Core2 Duo has
The E6600 is an amazing cpu for overclocking. If you have a good heatsink you can improve the performance a lot (as you can see, I got 3560Mhz with that cpu, but be carefull and take your time for doing it).
And yes, you will notice an important improvement with the E6600.
Am I corrwect in thinking you have 3x1GB at the moment and are going for 3x1GB&1x2GB? You will get much better performance from a dedicated matched pair of 2GB DIMMs as Moody says. Even adding a 1GB module would be better (although you would probably have to run all 4 at the specs of the slowest)