If you are buying parts in the uk overclockers.co.uk is great and has the best stock also Scan and Ebuyer are good but they don't always have things you want right away as they don't keep as much hands on stock.
I've got a Q6600 and its a great CPU, I've yet to push it to its limits and trust me I have been trying. On the graphics card you want to go with something new, for things like colour reproduction and image quality ATI have always been the best. But the new Nvidia cards have CUDA support which basically lets them run as there own self contained mini-computer and run programs not just visuals. Its not that well used outside of the academic area and for game physics but it does have some potential for different fields and Nvidia cards are very cheap at the moment especially the still excellent 8800GT.
If you look in my profile at my system its something pretty similar to what I would recommend to you for the basic system, I built it as a gaming rig but in general its just a powerhouse PC on a sensible budget. You could put it together not for somewhere around £700 leaving you £800 for monitors and extra peripherals or using slightly better components.