SLI with 3 wouldn't be a good performance boost over 2 with the 3 rd slot being x4. The way I read the MB specs it only support 3 way CF, not SLI.
that being said 2 GTX 670's in SLi needs 170W (MAX) each + system power w/ CPU (overclocked) totaled running at less than 80% than the PSU max. 340+208 =548W so to stay at 80% of max you will need 700W minimum (you might get away with a solid 650W as I am using maximum's and you rarely will hit them)
these are all good mid-towers. My wife's and son's gaming rigs are built using the Fractal Design R3. I recommend it as a good solid quiet case. you will need some additional fans though to keep it really cool. I like the Zalman and the NZXT Phantom also.