You can check the CPU with the command sysctl hw.cpu64bit_capable ( 0 means no, 1 means yes) or by looking up the model (if it has a 'Core Solo' or 'Core Duo' CPU it's 32-bit only if it's a 'Core 2 Duo' - note the '2' - it's 64-bit).
Short answer: if your model of MacBook has a 64-bit capable CPU, you can run 64-bit apps under 10.5.8.
