Hmm. As for the case, go with a Bitfenix Prodigy. It's slightly larger than a normal mini-ITX case, but that's a very good thing for you - you can get better components. It also looks good so your wife won't complain.
For a motherboard, this is a good one: http://www.newegg.com/Product/Product.aspx?Item=N82E16813157312
After that, it's the usual fare - a Pentium G870 or ivy bridge i3 2220. Grab yourself a good video card (I'd go with a nvidia 650ti, for BF3, or a radeon 7770/7850), 4GB of 1600MHz ram with CAS 9, a small psu that's NOT modular, and a hard drive.