For desktop use, you won't be able to tell any difference between 64bit and 32bit use. You'll only see a difference in high performance computing with high CPU usage. Running a pae kernel will allow you to use all 4GB of your RAM.
I have an Athlon64 CPU with 4GB RAM. I can tell you this. The 184.108.40.206-pclos1.pae.bfs is extremely good. I gauge its performance by the number of points per day I get on the folding@home tasks I run. Before switching to the aforementioned kernel, I was running an .a64 kernel, optimized for the Athlon64. With the pae.bfs kernel, my points per day have nearly tripled over a two or three week period. Nothing else changed, just the kernel used. The pae.bfs kernel is the first one Texstar's done that combines the aspects of Con Kolivas's scheduling with physical address extension.
You can run 64bit Windows in VirtualBox if you have a copy to install. I don't think you'll see any performance increase there, either. I'm told you can run a 64bit OS in VirtualBox, even if the host is 32bit, as long as you have PAE/NX enabled in VirtualBox. I haven't tried it yet, though.
As to your /home partition, you should be able to use the same one, keeping all your data intact. At worst case, you'll need to move some configuration files and/or folders to be sure you get a clean setup. That's pretty standard praxctice, though.