It's suggested that you get 4gb of ram, but 6gb of ram will also help, but not too much. 6GB is "suggested" by most people to be the most amount of ram you'll need for gaming for a while. But if I had the choice definitely get the 6gb it'll be more future proof.
you will see no difference in gaming from 4GB to 6GB, in fact 3GB is often the most any current game needs.
From Tom's recent x435 and i7 test "Because the Core i7-920 utilizes triple-channel memory, we use three 1GB sticks for a total of 3GB of RAM. The dual-channel AMD platform will use two 2GB sticks for a total of 4GB. From our experience, the single gigabyte of RAM difference should have no effect on gaming performance"
what kind of memory is yours? ddr2 or ddr3?
If yours is ddr2 then running on dual channels (2x2GB) mode is good.
If you has ddr3 then running it with 3 sticks of RAM (i.e 3x2GB).
On games, you won't see any different, anyway 4GB is enough.