A good rule of thumb when moving to a dual core processor over what is already an arguably fast single core for gaming- you need to get a dual core that is just as fast Mhz wise as possible. The dual core will seem slower at most games right now if it is not clocked very close to what you had as a single core. In other words, if you drop in a dual core CPU clocked at 2.4Ghz, it is going to make your games seem like they took a small hit compared to your old 55FX at 2.8Ghz.
Games that can make use of a dual core will do better, and the rest of your system will be faster, smoother, but for raw game performance, take these words of advice!
If you go dual core, (and you should if you can find one reasonable) get something that will clock at least 2.8 as well, or you will feel like you did not get your moneys' worth.