I'm building a new computer, do I go for a E6750 dual processor or go for a quad core?
You should go for the quad core. The cost of a decent one is about the same as 2 E6750 processors but you don't then have to spend money on a specialised dual processor motherboard. That extra money can be spent on getting either a better single processor motherboard or extra ram.
The current dual cores can also be massively overclocked to give a similar performance to quad cores!
Also, Vista Home Premium will only support a single dual core processor. If you want dual processor support you need a minimum of Vista Business which will considerably push up the price.