ATI is working on its new driver that will boost performance in dual core systems. Sources close to ATI claimed some ten to twenty per cent performance increase with dual core CPUs
Source
Our conclusion after these brief tests is that the new driver is a good first step in the right direction, but that the benefit to end users is minimal at best at this stage. The real benefit will come in when game developers start working on parallelizing their code as much as possible.
Anandtech's Review
Since a lot of people are asking for reviews of Nvidia's multi-threaded drivers to compare it to ATI: Here's a Link