64 bit. There is no reason to run 32 bit Windows.
Your computer is 64 bit
1. New applications/programs are 64 bit
2. 64 bit Windows runs 32 bit applications/programs
3. 32 bit Windows can only use between 3 and 4 GB of RAM. If you have or plan to have more than 3 GB RAM then you must use 64 bit Windows in order to use all your RAM