I have just bought a brand new MacBook Pro with 4 GB RAM, and was wondering which Windows 7 version 32 bit or 64 bit I should install with BootCamp, in order to do Visual Studio development? 

Is Visual Studio 32 bit? Any pointers to the correct install process would also be appreciated.
['https://superuser.com/questions/58913', 'https://superuser.com', 'https://superuser.com/users/-1/']
No brainer: 64bit.

In addition to the ability to use more RAM and run 64bit apps, 64bit chips have other features the OS itself can use to get performance benefits that a 32bit OS won't know about. More registers, for example.

So even if 32bit Visual Studio is the only app you run, you're still better off running 64bit windows. Not to mention it gives you the ability to build 64bit apps.
While Visual Studio is 32 bit application, depending on what type of applications you're planning to develop, you might find it beneficial to use the 64-bit version. IIRC you can run and debug both 32-bit and 64-bit executables in 64-bit Windows but not in 32-bit Windows (as the latter isn't able to run 64-bit apps).