what kind of intense graphical work do you wish to use the comp for?
as for gaming, you actually won't even need more than 4GB now a day, let alone 12GB.
Best manufacturer for the components to be used on your built:
CPU - Intel, definitely, I would personally recommend getting a i7-2600k or wait for IVY BRIDGE processors to come, as they will have more cores.
Graphics Card - if I were you, I would go with NVIDIA for this one, as they have CUDA, which many programs utilize
Ram - Corsair, G.Skill, Crucial, Kingston, etc.. Those are the highly recommended brands. Be sure that you get the high performance models with high MHZ and heatsinks
Motherboard: Asus, Gigabyte. Really, what else? They are the best overclockers