About 3 years ago I built my first PC, my PC is now on about its 3rd major upgrade. Would never look at a stock pc again.
My advice , always be able to log on to the internet while building the PC for advice.
ASUS motherboards are the best I have used, software that comes with them makes them easy to overclock you can do it from inside windows, even for a doughnut like me.
My last upgrade consisted of a new motherboard , AMD 940 Phenom II and a an ATI 4890. Arma now looks like it should.
Good luck mate with the build. You will hit problems, but you come out of it smarter having learned how to fix it.