My build has a P8P67 motherboard (http://uk.asus.com/Motherboards/Intel_Socket_1155/P8P67...) and at the moment has 2x2gb of 1333MHz ram. Since RAM is so cheap I was thinking of upgrading to 2x4gb RAM. Another reason for upgrading is that while my PC runs the games I want to play absolutely fine. It can slow down a bit if I run a game with something else in the background.
The question I'm asking myself (and you guys) is whether I should get 2x4gb of 1333MHz (http://www.scan.co.uk/products/8gb-(2x4gb)-corsair-ddr3-xms3-pc3-10600-(1333)-non-ecc-unbuffered-cas-9-9-9-24-150v) or 1600MHz (http://www.scan.co.uk/products/8gb-(2x4gb)-corsair-vengeance-blue-ddr3-pc3-12800(1600)-non-ecc-unbuffered-cas-9-9-9-24-xmp-15v). My motherboard says that it supports 1600MHz (O.C). How easy is it to overclock it to allow it to run 1600MHz and even if it wasn't too easy, would it be worth it at all (the price difference between the 1333 and 1600 is pretty small, would any change in performance be small as well?)
Small is correct. You won't notice that much difference. I found my 8 gb kits for $35 recently at newegg and ordered two of them. Just your standard 9-9-9-24 gskill ripjawsX 1333 ram. I go for price; just be sure your vender will exchange them if needed. Asus can be picky about ram.