Windows 7 memory usage in my experience of installing the RC on my Unibody Macbook Pro wasn't much different, perhaps a few percent lower (4-6% difference on my computer) and Windows 7 and Vista with Service Pack 2 installed run similarly the same with 2GB of DDR3 RAM, which made a difference in speed. I can't justify a $199 upgrade to Windows 7 ($199 since I'm using Windows Vista Business) since there are minor differences and improvements compared to Vista, it feels like a expensive service pack, but if it was priced like Snow Leopard, then Windows 7 would be more appealing, but not so much since I only use it for games... I rather spend that money on DDR 3 RAM and Mac OS X Snow Leopard.
Also, installing Windows 7 will not make much of a difference with Vista with graphics performance. If you
check the results comparing the gaming performance with Win7 and Vista, there isn't much difference.