I run Sonar X2 x64, and ironically, some plugins work better for me in 32-bit than in 64-bit.

Rapture, for instance. The 64-bit version crashes Sonar consistently. The 32-bit version works fine.

It's definitely best to have a 64-bit OS and DAW. But from an instrument perspective, 64-bit is only important if the 32-bit version proves unstable, or if that instrument itself uses a lot of samples. 64-bit is also important for ReWire since it can be used to tie full sequences from Reason or Ableton into Sonar.