a) not every single app has to be 64-bit
b) Vista x64 has high compatibility with 32-bit apps
c) the number of 64-bit apps will only increase
Sorry, I wasn't meaning to imply that Vista x64 won't run x86 applications, rather that you won't get the performance benefits if all you're running is x86 versions of your software. The problem is that many people look at the numbers and assume that because 64-bit is 2 x 32-bit, that everything will run twice as fast. I've had managers here asking me to upgrade their laptops to Vista x64 because it will double the speed they can surf the web!! ;D
True, the number of 64-bit applications will increase, but until that happens you're stuck with the older versions of whatever software it is that you use.
One of the interesting stipulations Microsoft has regarding certification of drivers is that any device a manufacturer wants to certify for Vista must have a 32-bit and 64-bit driver. Likewise, software must be able to run on both. It doesn't need to be 64-bit, but it needs to properly run on Vista x64.
The worry there is that unless the manufacturer for your device wants to go down that route, you're stuck. There are thousands of legacy devices out there that will never get x64 drivers, but then if you don't plan to use one it won't be a problem.
Vista x32 is a desperate measure, and one I'd avoid. If the Macbook Pro will have 4GB, you will need to use Vista x64 to properly use it all anyway.
Agreed, 32bit Vista is a dog's breakfast! ;D Technically though, it can address up to 4Gb of (total) RAM (ie, incl. video RAM) although getting it to use more than 3Gb is a royal pain, and yes, x86 apps are still limited to 2Gb. Problem is that benchmarks tend to indicate that Vista x64 doesn't really start to outperform it's 32-bit cousin until you start adding
more than 4Gb of RAM (seeing as the x64 version of the OS has higher overheads anyway).
In the end it really depends on what use it'll be put to. Personally I'd go for a Windows XP 32-bit install. ;P