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.
Visual Studio is a 32 bit application only, so if that is the major reason for installing windows, 64-bit will not give you any additional features, other than it will be able to handle all 4 GB of memory, whereas 32-bit will only give 3.2 GB or so.

See this question for details about general 32-bit vs 64 bit systems: 

* [32-bit vs 64-bit systems](https://superuser.com/questions/56540/32-bit-vs-64-bit-systems)