XPS 420 RAM upgrade

Hi All, so have just upgraded OS to Vista Ultimate x64 and am now looking to increase my RAM from 3GB of DDR2 667Mhz to either 8 or 16 GB of DDR2 800Mhz.
As there are only four RAM slots available does anyone know if 4GB modules are supported by the motherboard?
If they are does anyone know where they are available at a reasonable price (UK)?  All the ones I have seen have been £80-100 ($120-160) per 4GB module.

System specs:
Intel Q6600 2.66Ghz
Standard Motherboard
3GB 667Mhz Ram