This has been beat to death over and over, but get a good PSU if you plan to mine for any real length of time.
Generally plan for 200W / card + 100W for system, and you'll have a figure to shoot for under worst-case scenarios. Keep in mind that most PSUs won't push their full rated power output, or at least not for very long without burning out. Research the measured power usage for the cards you plan to use, and if you think you may ever upgrade, ensure you have the capacity to support higher-end or more cards. 6990s will use ~375W each, for example.
Spend the extra to get something that's at least 80+ and 100W more than you need, and you'll likely never have a problem with it. In the case of PSUs, the more you spend, the more you save over time. A $100 80+ Gold supply is going to waste far less power at the wall than a $20 generic PSU; in the long run the cheap PSU is going to cost you more in utilities.
As for the other components, go as cheap as you wish long as they're well rated, and remember to leave yourself room to upgrade.
 I realize I totally glossed over the initial MB question, IMO get something that will fit you at least 4 cards, either in x16 or x1 slots, but beyond that point it becomes cumbersome to supply power to all your cards in a single system reliably!