Realistically you wont see that big a difference. Your main issue is that you have 3GB. that means you cant be running in dual channel mode.
How are you configured ?
This is how I see it.
If you have 3 1GB strips put them to one side as spare's and get 2 x 2GB DDR3 1600 strips and put them in dual channel mode.
If you do have a 2GB DDR2 strip then check the price for a second strip the same to put with the one you have in a 2 X 2GB DDR2 set up.
I have not checked prices for a good while now but you could well find that its going to work out cheaper or not that much more expensive to go with a 2 X 2GB or even a 2 X 4GB DDR3 1600 set up.