I don't think there's a need to change the processor at all since a quad core at 3.5Ghz is pretty fast already (by today's standards). It will also be an expensive excercise since that requires new motherboard and DDR3 memory If using it for professional applications (such as 3D rendering software such as Maya, 3dsmax, Truespace, etc), then the hexa core makes sense. Otherwise for most everyday applications and games, the quad core is way more than sufficient. If you want to upgrade then wait for the next iteration (or next generation), or until you feel your machine is inadequate (which is very unlikely).

Already considered very fast already: Core 2 Quad 3.2GHz = Core 2 Extreme QX9770 > Phenom II X4 965 (in most applications and games).