OK, $400 without an OS or GPU gets you into a whole different ball game:
Mobo: ASRock Z75 Pro3, $85
These are great cheap boards. Asus would be a little better on features in the BIOS, and have a little better quality onboard parts, but this should get the job done very well, having all the necessities of a high end board, without the costs.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157304
CPU: i5 3450 3.1GHz, $150
this is a locked CPU, but you should still be able to do a Turbo OC up to ~4-4.3GHz, and even at stock it has more than enough horsepower to max out most games. Run stock cooler for now at stock settings, if you want to play with OC later be sure to buy a Hyper 212+ or Evo first.
http://www.microcenter.com/product/388579/Core_i5_3450_31GHz_LGA_1155_Processor
RAM: 8GB DDR3 1600, $43
I personally use Corsair in all my builds, but gSkill is also pretty good if you find some on sale
http://www.newegg.com/Product/Product.aspx?Item=N82E16820233202
HDD: 1TB 5900rpm, $70
same as before, this is really meant to be a storage drive, he should plan on purchasing a SSD before long for the system drive, and then use this as a storage drive, but it should make an OK system drive for now.
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148764
DVD: DVD burner, get whatever is cheapest after shipping, $19
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151255
Case: mid-tower $20 after rebate
Again, something cheap with a bottom mount PSU to be upgraded later (don't spend money on looks until you have the internal parts that you really want).
http://www.newegg.com/Product/Product.aspx?Item=N82E16811156255
PSU: Antec 620W $70 after rebate
power supply big enough to do xFire with a 2nd GPU down the road
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371059
Total Cost: $457 (sorry, went a little over budget)
To get back into your budget range there are a few things you can sacrifice, but for ~$60 I think the added features and upgradability of the above system are more than worth it:
Get a smaller PSU that can only support a single GPU (-25)
-this would not hurt performance, but would limit future upgrades to not support xFire
Go back to a cheaper MSI motherboard (-20)
-Still a good board, but no overclocking, only a single SATA3 port and total of 4 SATA devices, only 2 ram slots, and a general lack of other nice features. Again, it does not hurt the stock performance what-so-ever, but it seriously limits future upgrades. However, I love the UEFI on this board, and MSI (while I would not use them in a 'high end' system) makes excellent cheap boards.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130654