WoW has very limited multi-threaded support. For two boxing, you will not see much of a performance gain between the dual core and quad core processors. There will be some, but real nice gains aren't seen until you are 4+ boxing. Just make sure you are assigning your threads to the different cores, properly.

At first glance, I'd say ADD MORE RAMz, but you already knew that. Up until today I have always recommended DDR2-800, because it's so much cheaper. But that DDR3-1066 you linked is a good deal. Will you notice performance gains between the two? Probably not, but it's a nominal upcharge for having something that is slightly better.

Judging by your current specs, I would take an educated guess that your current mobo couldn't handle a 64bit quad core processor and 8gb ram. Be careful choosing, check the manufacturers page for specific compatibility.

Your current vid card is on par with the rest of your system: three years old. If I had my choice of the 4 cards you listed; I'd take the GTX 260.