I would wait till you can also UG to CS4 or CS5; as they are designed to take advantage of 64 bit.
XP uses the ful 4 GB of memory, there is no extra 1 GB when using Win7-64.
Do a Google search with the keywords "Windows only reports 3GB RAM" and you
will see my point. I know that on the Windows website it does say that XP
will use 4GB RAM. Also, if you look in System properties Windows XP only
reports 3GB RAM. I am hoping that Windows 64bit will use the full 4GB RAM.
Maybe it will not unless I invest in more ram. I will have to do more
research on that issue. Anyway my Epson printer only has a 32bit driver so
until I can upgrade that I have to stick with what I have.
Of the 4 GB that XP and Vista-32 and Win7-32 can adress, XP reports it differently than Win7. However in both cases part of that 4 GB adress space is used by Windows for mapping video memory and houskeeping tasks.
I would upgrade to Win7-64 Pro only when you add additional memory (to 8 or 12 GB memory). You will see a performance boost and better stability. It is useless to upgrade to Win7 without additional memory.