First I want to say I wish I knew about this site 8 years ago, would of helped!
Anyways, my starting point is the CPU which will be the i5-2500k(Read this is the best bang for buck). I am planning to OC but not sure what mobo to get as I'm also thinking of going with 2 GPU's (GTX550ti). Altough I'm reading that I should just buy 1 GPU. Torn on this quite a bit.
Agreed, if going Nvidia, get a single 560Ti. You'll be happy with it. If going AMD, get a 6950 or 6970. Also, 8 GB of RAM is plenty - no need for 16 GB. The power supply you chose is excellent. Be sure to get the Rev 2 PSU. As for a MB, there are lots to choose from, Gigabyte and Asus being the top two choices. Pick one with lots of power phases to assure smooth overclocking, and just check the port arrangement to be to your liking. The field should narrow itself down considerably.
One thing I would recommend on your mobo is UEFI Bios. I've personally never used it, but it looks so much more handy and prettied up, plus its pretty much the future of Bios so there isn't much reason not to. My next build will definitely have it.
If you get an i5 (and the P67/Z68) do you have a choice on UEFI? My i5 builds had it (Asus boards) and it worked fine. Nothing really special though for what I did, I felt no great loss when I went back to the 'classic' BIOS in my X58 system. Still probably good to have though.
So I ended up going to Micro Center in Silicon Valley which had some 2500K advertised for $179.99. Unfortunately I was too late as they were sold out. The good thing is they also had the i7 2600k on sale for only $279 with an additional $50 off any motherboard if I chose. I did end up spending a lot more as I was able to sell a couple of things that helped my budget ou. Here it is..