For gaming, a significantly faster single core will perform better than a slower dual core, since most games are still single threaded. In the future, when the landscape of the gaming world shifts to take advantage of SMP, then we will likely see more benefit in dual core/ multi-core gaming. When UT2007 comes, that's one of the first big titles that will have full support for multicore systems.
If you are talking about most games today In general; even between a 2GHz Turion and a 2GHz Core Duo, Core Duo's FP performance is not its strong point architecturally, having only one FP unit to Turion's three (FADD, FMUL, FMOV). So for first person games, you will probably see the Turion perform better at the same clock speeds on the physics engiene (which basically calculates the change in trajectory of objects when acted on by different forces in the environment, a classic FP rich operation), and AI algorithms (such as probablistic MiniMax, which also uses a large amount of FP operations) . Large scale scenen renderings in games, however would favor the Core duo, due to its good ALU performance, and especially its large L2 cache which would provide a nice little extra boost.