I doubt that you would see a difference in terms of gaming performance. However they are NOT interchangeable on the same motherboard. The slots are physically different, and DDR3 will not fit in a DDR2 slot and vice versa.
gaming doesnt take much use from ddr speeds, if you have ddr2 and want to upgrade to ddr3 and leave the rest of the specs the same, cpu, vid card etc, you will see no difference.if building a new system ddr3 is the way to go, its cheaper and is only supported by recent motherboards. The only time you will really se a difference in ddr speeds is when using integrated graphics which uses that ram, in that case the faster the better, but anyone gaming would use a dedicated vid card, so its a non-issue.