If you are planning to install Windows 7 on your current computer, 32-bit is recommend because of drivers and software compatibility issues.
If you are buying a new computer, select 64-bit. If you are assembling a new computer, make sure all hardware you purchase have 64-bit drivers.
32-bit software and drivers will not work in 64-bit.
64-bit software and drivers will not work in 32-bit.
Even your printer, scanner and mobile will stop working in your 64-bit computer if they do not have 64-bit drivers.
Its impossible to find 64-bit drivers for old hardware.
It is recommend to switch to 64-bit when you are buying a new computer. Starting late 2009 and from 2010, 99.99% of software and hardware will support 64-bit.
Note: Retail versions of Windows 7 will include both 32-bit and 64-bit discs.