Windows 7 64 bit installs both a 64 bit OS and a 32bit section as well so there are no compatibility issues. Your 5D will work with the 64bit OS without a doubt, because it's still supported in the latest EOS utilities and DPP which are installed into the x86 program section (32bit).
Adobe CS4 is a 64 bit program but that installs a 32 bit version as well so you can use old plug ins.
4GB is the maximum 'address space' not RAM, the most of that is something like 3.5GB or less. Given that the OS will occupy quite a portion of that, it doesn't leave a lot for photoshop projects. It's not unusual to see 1GB once a few layers are added. You haven't said what your current system is, but if the video card can use the Ram then you have even less space to work with.
There's a fair pros & cons article
here As it says, the only thing you need to do is check that drivers are available for your components, and go ahead!