1.32-bit builds are limited to around 2GB of data. In general you should use the 64 bit builds. The 32 bit binaries are ok for replica set arbiters and mongos' but not for production mongod's. See here for more information.
2.The 64-bit legacy build lacks newer features of Windows that enhance performance. Use this build for Windows Server 2003, 2008, or Windows Vista.
3.Every download has a corresponding md5 checksum, which can be accessed by appending the .md5 to the end of the download url.
4.The MongoDB server (mongod) must run on a little-endian CPU, so if you are using a PPC OS X machine, mongod will not work.
5.See here for more information on MongoDB version numbers.
6.Included in each distribution are the MongoDB server, the MongoDB shell, backup and restore tools, import and export tools, and a GridFS tool.
7.See MCI for details on ongoing builds and completion times.
8.MongoDB Enterprise provides advanced security, on-prem monitoring, SNMP support, and more.
9.The Production Notes page contains a full list of operational recomendations for configuring and deploying MongoDB in production.