That's an interesting question. My gut reaction is to say 2GB DDR3 almost immediately just because of the speed and DDR2 is beginning to go out of style, but what's the context that your asking this question in?
Are you looking to upgrade your RAM? If so, you'll need to upgrade based on what your system can handle.
crucial.com is a great website and will help you pick out new RAM if this is what you need.
It all depends on your configuration, 1gbx3 ddr2 vrs 1GBx3 DDR3 see they are entirely different. the ddr3 has way higher bandwidth that the ddr 2 which in turn means more speed, so stick to DDR3 if you want the best perfomance
i think moricon is spot on this time.. yes, ddr3 is faster but it isn't readily noticeable unless you overclock the ddr3.. if you're running a 1gb/512mb/1gb/512mb configuration for the ddr2, it should be better since you have 3gb, not 2gb.