From what I have read, the 3650 is supposed to have 2000 Mhz memory on a regular-size PCB (as opposed to the current GDDR4 version of this card). The 2000Mhz frequency is good for a slight performance increase.
More importantly, If you have Vista, the 3650 is supposed to be DirectX 10.1 compatible.
Also, as the 3650 will be built on a 55nm process, there is a better chance for factory-overclocked versions.
We can expect the price of the 3650 to be lower, because of the cheaper 55nm process and the performance difference with the 3850.
As the 3650 is supposed to come out towards the end of january, there is little risk involved in waiting until then. The worst that can happen is that the price of the 2600XT will continue to fall.