Will give you a quick reply plus bump as others will chime in soon I'm sure.
For me and a lot of others here, windows 7 64bit (with reaper 32bit or 64bit) has been a great experience. As stable as xp (which is pretty good by sp2) but with lots of handy bells and whistles. I could never go back to xp now.
You may also be likely to find drivers for win 7 64bit than xp 64bit for new hardware too (although this is slightly speculative on my part)
If you have any specific questions ask away but I would give win 7 64bit a go, it will handle your 4 gig pretty well although it does eat up a little more than xp (others might be able to give more detail)