I would, as was already suggested, "downgrade" to 32-bit Windows, and if you need access to that .5GB of memory you'll lose, you can modify your 32-bit OS to recognize more than the 3.5GB of memory. I have done it before, and I would provide a link, but it was a couple years ago when I did it last, and I'm sure if you Google how to do it you'll come across it no problem. Works with Windows XP+ IIRC.
Oh, and an upgraded CPU is also a good idea, just make sure it's compatible with your motherboard before making any purchases.