Building your own PC is a piece of piss. Its so easy nowadays, everything just slots together.
If you're going to build, which you should, start with which Processor you want and take it from there. The CPU will decide which socket motherboard and which socket ram. All new motherboards will be PCIE 3.0 so get a GPU to match to ensure max bandwidth. GPU's are quite wide ranging (price and performance) but if you've got some cash to blow you can get a GTX780SC from scan for £400, and that beast, matched with the right CPU to avoid bandwidth bottlenecks, will run anything. If thats a bit pricey then anything over 1000 cores/1GB ram + will do well on current games.
Motherboards come with pretty much all the bells and whistles that you used to have to add with cards so its much much easier to build the damn things now.