I start at the processor.. Then find a mother board that supports that processor, enough ram slots, etc to meet my needs. Then I pick out my ram, video card, etc. all the while keeping in mind my budget..
This time 14 months ago I settled on:
Intel Core i5-4670 Haswell Quad-Core 3.4GHz LGA 1150 84W Desktop Processor Intel HD Graphics BX80646I54670 (The K version doesn't support some virtualization options, and quite frankly im not going to bother to overclock anyway.
ASRock Z87 Extreme6 LGA 1150 Intel Z87 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboard - for various reasons, but mainly price, rather had had an ASUS, but this has been nice.
So the processor works in that board, now I need ram.. Just a matter of picking out the right ram. Reading the MB specs will typically tell you what you need to know on that end..
Crucial Ballistix Sport 8GB 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Low Profile Desktop Memory Model BLS8G3D1609ES2LX0
From there.. I used my old case and replaced my PSU with this
CORSAIR HX Series HX750 750W ATX12V 2.3 / EPS12V 2.91 SLI CrossFire 80 PLUS GOLD Certified Modular Active PFC Power Supply New 4th Gen CPU Certified Haswell Ready
Used my old Video card, etc etc.
Added this to replace my old SSD
SAMSUNG 840 Pro Series MZ-7PD128BW 2.5" 128GB SATA III MLC Internal Solid State Drive (SSD)
I plan to replace my video card at some point... Still haven't.
But today I would get something totally different.