Unless you plan on doing other stuff with the system, I would go with a much cheaper MB and CPU... Even your PSU price is too high :]
Go with an AMD 145 CPU, a MB that costs no more than $100 (yet has enough PCI slots), and maybe $230 for the PSU (EVGA G2 1300's or Rosewill 1300's are around that price)
If you are comfortable with Linux, a mining distribution (like BAMT) is free, other than the cost of a 4GB or larger USB Drive
Riser cables can be purchased for $10 to $20 a piece depending on if you want powered or unpowered... 1x or 16x
Of course, if you also want to game / use the system as a "normal" computer then a lot of this doesn't apply... otherwise it's all about getting the cheapest parts that still work reliably (other than the high-end GPU's of course)
As for profitability, that's something that is much harder to predict. A few weeks ago a "normal" rig of 2.5 MH/s or so was making >$50... a few weeks before that you were lucky to make $12 a day... I think $15-20 (not including power costs) is perfectly reasonable, however, I say that with not much confidence