Quad core
-Not much more expensive than a Dual Core (you can't really buy a single core any more? lol?)
-Games are starting to use not only 2 cores, but 3 or 4. (Fallout 3 uses up to 3 cores, SupCom can use all 4).
-Multi-media applications have been using quad cores for years.
-Tech is just as stable as existing dual core CPUs as it is just 2 chips on 1 die.
There is no reason to choose a single core processor, Dual cores are cheap, widely available and work great. Quads are even more useful if you can afford the extra $20 for one.