OK, I'm going to explain this as best as I can, and why you should take one over the other.
Single Core processors are no longer the standard in CPUs. Dual Cores are. Dual Cores are becoming utilized in more and more games, and in the future, bearly every game will take advantage of a Dual Core processor. Yes, some Single Core processors can out perform some Dual Cores in some games, however, this is not because of the Dual Core processor, it is because of the way the game was designed. Games that were designed last year were not being made to run with a Dual Core system. However, this year games are goign to be made to take advantage of the Dual Core processors.
Single Core processors are dying, and rather fast I might add. With the launch of AMD's Socket AM2, they will with it launch 6 new Athlon X2 Dual Core processors. Not to mention, Intel is also launching a new product line duped 'Conroe', all of which are goign to be Dual Core. Then to top that off, at the end of this year we expect to see Quad-Core processors. So, with the way technology is going, Single Core processors are dying at a very high rate.
The time to purcahse a Dual Core processor is coming. By the end of this year, we expect Dual Cores to be extremly affordable and very much worth the purcahse.