It is game specific but generally speaking you should prioritise the amount of RAM over the speed of RAM. Having enough RAM that your PC doesn't have to use its paging file (on the hard disk) is better than having faster RAM but less of it and using the paging file. So I would take 4GB of DDR2 over 2GB of DDR3.
That said, CPUs that were in production around the time of DDR2 are significantly poorer performing than modern DDR3 era CPUs. You will also find it easier to get hold of large DDR3 DIMMs than you will DDR2.