Installing a 64 bit OS or keeping a 32 bit OS?

[Jef Driesen]

Currently, I have the 32bit version of Ubuntu installed on my laptop (an
Intel Core 2 Duo), but I'm considering reinstalling with the 64bit
version. Are there any disadvantages? Can I still run 32bit
applications? Can I upgrade to the 64bit version or do I need to
reinstall? Can I have them both side by side, without doing two
completely separate installations (dualboot)?

The laptop is mainly used for software development, so I would like to
have a 64bit environment for testing my code. But I still need to be
able to compile for a 32bit environment (and preferable test them as
well). Is that possible?

I also use the laptop for playing some multimedia. Will that be a
problem? As far as I know, it's not possible to use w32codecs on a 64bit
Linux? Are there any alternatives for playing wmv, flash,... ?