My m-itx build is pretty much identical to that spec, apart from the fact I went for an i5-760 and a Vertex 2E.
I've overclocked my cpu to 3.4ghz (155 x 22) at 1.26v. I've left turbo enabled so I actually get 3.87ghz in lightly threaded apps. As you may have seen in my thread in the SFF section I'm using a corsair H50 for cooling. It required some modding to the case to fit, and it restricts your max GFX card length but it works brilliantly, my cpu idles at 25-30c (I have speedstep enabled), and doesn't get any higher than 55-60c when under full load in LinX.
I have tried higher clocks (without turbo enabled of course) but I just can't get it stable. I can boot and get in to windows just fine at anything up to 4.2ghz, but I can't complete more than a few passes of LinX without hard lock-ups, no matter how silly I get with voltages. My current clock is rock solid stable, plenty fast, and runs nice and cool so I'll leave it be
The GA-H55N supports an 1156 i7 but you can't use a 6gb kit because 6gb kits are triple-channel, the GA-H55N is dual-channel and only has two memory slots. You'll have to use 4gb or 8gb.
I'm personally going to be buying a Gainward GTX 460 gfx card soon, it's a short card at 8" long and has the PCI-E power connectors at the top of the card so they won't foul my H50 rad.