Hi,
I've just been through the process of specing and buying a new PC and I'm currently awaiting delivery of my new purchase. I've just recently got a DSLR and wanted a decent computer to run Adobe Lightroom on and possibly Photoshop at a later date.
After a bit of research I decided that a system based on an Intel Sandy Bridge processor would be ideal and finally went for an i5 2500k as it's a nice compromise of power vs price and can be easily and safely overclocked to achieve greater performance.
Again after research and reading plenty of reviews I decide to get quotes from 3 companies, Dino PC, Chillblast and Scan. There wasn't a big difference in the prices they quoted but I eventually went for a system from Chillblast as they where the most helpful when assisting me spec a suitable machine.
I'd suggest setting yourself a budget and then speaking to a few firms discussing what you need the PC to do and get some suitable quotes.
Let me know if you need anymore info.
Mark.