psu, grab yourself a corsair builder series 600w kick about the £50 mark, don't waste money on cheap power supplies if you learn anything from building your own pc is don't skimp on the psu.
next is the graphics card, nvidia 550ti as minimum, amd are usually cheaper and you should be able to get a 6850/6770/7770 for around the same price as each other.
psu/gpu are the two things you make as little compromise on as possible for gaming, the psu is to be reliable and actually do what it says on the side of the box, and the graphics card if skimped on will be your biggest disappointment.
the fx4100 is a nice cpu nothing wrong there a decent quad core.
if your looking for places to cut corners try not to, consider getting a single 4gb stick of ram initially and then grabbing a second a month or 2 later.
gaming can cause a lot of stress on a system nothing like what stress a full blown rendering station will do or a vm ware station goes through but still it gets toasty, so cooling is important make sure the case has sufficient cooling options or the option to add more.
think of it this way, you can always grab additional parts later granted some are more key than others but so is getting value for money a cheap psu can cost you more due to failures where a quality one will actually last you years and give you peace of mind.