Justere, a few high-end DDR2 modules (such as some Kingston HyperX) are capable of stable operation in 1T mode. However most DDR2 RAM is not. If you are able to get your RAM to function and be stable, then you may see a slight increase in performance. Most of the time, though, the only way to achieve stable operation in 1T mode with fast DDR2 is by decreasing the speed and increasing the latency of the RAM. So usually it is better to just use 2T mode as you will usually be able to increase the speed and decrease the latency without many stability issues, resulting in better performance overall.
Although, if you have some very nice RAM that is stable in 1T mode with a high speed and low latency, then by all means, go right on ahead and enjoy the benefits of 1T mode. All the rest of us will envy your super speedy RAM. And if that is the case, let me know what RAM you are using so I can get me some of it too. B)