I would thought that as most photo people you would be looking at MAC for this.
If you want to stay 'PC' then you are probably looking at multi processor setup. Be careful with Gfx cards as teh most expensive will probably be more geared for gaming than for what you want which will be colour accuracy etc. I would be asking this on teh photography forums TBH as they will have much more experience and advice in this matter.
Monitors - make sure you budget for teh colour calibration tools as these can themselves be a few hundred quid on their own.
RAM, I would reckon 16 gig should be plenty
You also should spend some time playing with Gfx tablets as again this is probably going to be needed and is another thing that can cost a lot more than you think.