Bottom Bracket compatibility?
You can indeed replace the square taper cartridge with an external bearing bottom bracket and fit new cranks.

Shimano's system is HollowTech II, SRAMS's is GXP, there is also FSA's MegaExo. I believe that the bottom brackets and cranks of each of these systems are not cross compatible.

Note that many bottom brackets are labeled as either 'road' or 'MTB'. Road brackets are typically for 68mm wide BB shells while MTB brackets are typically for 73mm shells.