Also, you can overclock the dual-core to roughly the same frequency as you can overclock the single core, therefore having high clockspeed, and still have the benefits of dual-core.Because the 64 3800 has a faster clock it will be faster in alot of games that currently can only use one core.
As more games arrive that can use multiple cores, the X2 3800 will have an advantage.
The X2-3800 runs at 2x1.8GHz