For a nice table of what lvl of powers you can USE (eg, a wizard can have prepared) every day, see CH8 in the DMG. There's a nice table there, which really would've been placed a lot better in PHB. (Remember though, that you don't have to appoint powers of exactly that level, if you like a lower power level (like sleep instead of another lvl2x daily power) you can use that one too.)