When you hear terms like 16 bit and 32 bit applications, or operating systems, the 'bit' is referring to the length of a command (instruction if you will).
So.. if you have an application that is giving your computer's browser instructions that are 32-bits in length, and you're browser is only able to understand instructions that are 16-bits in length.. you can see where this might be an issue.
Safari should have a 32 bit version out, and it won't hurt your Mac at all. As far as where to change this... It will probably be under the 'Preferences' on the options menu. There is probably a 'Help' as well, but I'm not sure.
You may want to get on a Mac or Safari board and ask, as there will probably be a lot more people that will be able to be very specific on where to go.
I'm sure there will be some here as well.. but percentage wise, it'll be a lot less.
Hope this has helped some.