I take a contrarian view on these things ... I built a few PCs myself. Yes, putting together the parts is DEFINITELY not hard and is a cool, feel-good-about-yourself kind of project. However
1) If something goes wrong (e.g. the aforementioned mobo / g-card conflict) it can be nightmarish to figure out what the problem is and a huge timesink
2) Building a fast, great-graphics, huge-storage machine within a budget is doable. But having that machine that is also cool and quiet is a different challenge altogether. When you're building your own thing, you'll end up being conservative on power supply and such (since presumably you don't want your rig to catch on fire) ... but then before you know it your rig sounds like a vacuum cleaner when it boots up ... and maybe there's a little rattle because you couldn't get that HD mount seated QUITE right ... or you're a LITTLE worried the CPU fan is going to slip out of place and slice some wires open ... or something
So to me when you buy a pre-made machine, you're buying
a) a proven, integrated machine
b) something where usually you can get some idea of whether make your game room sound like the deck of an aircraft carrier when it's on
c) customer service/support/etc.
I'm personally partial to Dell/Alienware but I realize I'm getting ripped off there. I have not heard good things generally about the glossy-magazine-ad-iBuyPower type places. I have heard that Velocity Micro makes good stuff that's at the sweet spot of build quality, value, and performance, and the Z40 has particular good reviews. For $1338 I think you can get the components Xaveor describes above (i5 2400 + 4GB RAM + HD6850 graphics), plus a 23.6" monitor. I expect tax & shipping would tack on more cost though, so might need to shave a little to get it under $1300. But it's close. And you wouldn't have to mess with anything yourself.
http://www.velocitymicro.com/wizard.php?iid=193
My personal "incremental dollar" advice ... getting a solid state hard drive is the most impressive gaming performance boost I've had in a machine in over a decade. I am always first in BGs, load screens are most instant, it's truly incredible. Now SSDs do involve some big tradeoffs ... namely $200 extra for 1/10 as much capacity as a typical 7200rpm 1TB HD. But they really are like magic as far as I'm concerned.