The 32-bit memory subsystem can't handle anything beyond 4Gb without PAE and PAE is basically disabled in consumer versions of Windows since most 32-bit device drivers can't handle it.
I remember back in win98 and XP, we had to fiddle around with the PAE and even run special .bat files to have use of more ram for our autocad and load calculating programs, back when I was able to work...... kind of miss working and doing that stuff, not using the brain functions using those programs/software for over 6 years , I can see and realize the toll it has taken on my memory.....
back to the topic:
save your money for a new build, go ahead and upgrade to Win 7 64 bit ( the home premium version minimum, but I would recommend the Win 7 pro 64 bit minimum personally, since home premium tops out at 16 GB of sys ram )
I have some spare sticks of 1 GB and 2 GB DDR2 memory , if you need a couple.......... shoot me a pm
TC