Photoshop :: Windows 7 Memory Use - CS-4

My XP machine is maxed out with 2 gig of chip memory (AMD dual core). Please, no laughing. It does run Photoshop CS 4 at an acceptable speed with 500 meg. PSD pictures. Apparently, there is a way to set up the system drive to dual boot to Windows 7 or XP with a "virtual hard drive" set up. How much less memory does Windows 7 use as compared with XP Home? With the early order half price for 7 Professional, it might be a way to give Photoshop some more memory (5%, 10%, 15%,?%) without having to get a new mother board and new memory chips at a far lower price. I am using Photoshop in a non-work environment. My full time job, hasn't given me a pay raise, in the last 8 years, that matched the increase in health insurance cost, I am willing to accept less than optimal performance boost at an affordable price. And, as I'm paying off a month old Epson 3800, every little bit of savings helps. I am aware that I might/will have to reinstall Photoshop under this set-up.