Depending on how much hardware you have in your computer running windows 7 64bit is still worth doing even on 4 gig. You can still run 32bit reaper in windows 7 64bit and still be able to access more ram than you can in xp.
At the moment if you have your motherboard set to 32bit mode then chances are the graphics card and a few other components of your computer are taking away some of that 4 gig in the form of memory address space and the only way to get it to be truly 4 gig is to switch to 64bit mode in the mobo and run a 64bit operating system.
Now I've been told that not all motherboards behave like this and some do not eat up memory so if yours reports the full 4 gig then you don't need this and you also don't need it if your projects don't go over (i think) 2.6 gig (or a bit more with the 3gig switch).
I might be wrong in some places and i'm happy if someone wants to correct me but that's what I got out of the whole experience anyway!
