The bottom line is at the moment DDR-3 is not yielding any real-world performance gain, especially on our focus topic - gaming.
The thing to remember is though, it will never really yield any gain, much like DDR and DDR-2 memory technologies. What gives us the gains are the pieces of equipment that surround the memory and what they can do - such as, processors with a higher front side bus hence offering higher 1:1 clock speeds. The memory itself is not giving us the direct gain but coincided with the newer processors, front side bus or the new Intel X38 chipset (which is going to offer more bandwidth), we are able to see those all important total gains in system performance.
If you've got the money and want to go DDR-3, then by all means go ahead and build a system based on this memory technology. However, we would suggest you wait for the X38, which is designed more for it (and will offer better performance and overclockability), as opposed to the P35. Although, on a whole, it is the future and you're going to be buying it sooner or later anyway if you're an upgrade maniac, like most of our readers.
Then there is of course the people who buy the best because they want their PC to be future-proof for the next few years - in three years time you might want to chuck some more memory in the system and DDR-3 is more than likely going to be easier to get your hands on compared to older DDR-2.
If you're on a P965 chipset at the moment with a decent graphics card and CPU and want to get some more frames, don't even contemplate a new motherboard or DDR-3, you are far better off thinking about maybe an extra 2GB of memory and / or a new graphics card upgrade. Prices will fall on DDR-3 just like everything else, more modules will pop up and higher front side buses will be up for the taking. DDR-3 isn't going anywhere but forward thought there is no reason for your average Counter Strike Source player to run out and spend big dollars upgrading to a DDR-3 system.
Remember, buy smart and think how your wallet would want you to!