Good - 1x120mm front intake and 1x120mm rear exhaust
Better - 1x120/140mm front intake and 1x120mm rear exhaust and 1x120/140mm top exhaust.
Excellent - 2x120mm or 1x140/200mm front intake and 1x120mm rear exhaust and 2x120/140mm top exhaust or 1x200/230mm.
Also many have a host of different side panel mountings.
I'm refering to higher end mid-tower case.
Some Full towers can accommodate 3x120mm front intakes and 2x120mm rear exhaust and have a mix of side panel and top exhaust capabilities.
As far as power saving go fans are the least concern since they use next to nothing.
Concentrate power savings on the psu efficiency,gpu,and display monitor.
^+1 to Davcon, nicely given but many do differentiate on this. Also, do look at the fans CFM measure (cubic feet per minute). Those figures come with the fan specifications. An ideal case should have equal intake CFM and equal exhaust CFM. Since the world we live in isn't perfect, we should look to balance this as much as possible.
I'll agree that the "ideal" is equal intake and exhaust fan capacities. Well, almost - actually I prefer slightly more intake capacity so that excess air can leak OUT of the case, not in, because inflowing air carries dust. HOWEVER in practice you cannot figure out the air balance from fan capacity specs because ALL of your INTAKE fans ought to have dust filters on them. The specs for the fan cannot take into account the reduction in air flow caused by the filters. So what you really end up doing is setting up fans, the using some tracer technique (some have suggested the smoke coming off an incense stick) to test the air flow direction at case leakage points. Adjust your fans if necessary.