That 820MB figure is the size of the page file. As you can also see in the screenshot, there is more than 600MB of free physical memory. Windows uses the page file even if there is sufficient RAM. It's how it works. It may look inefficient, but as can clearly be seen in the tests between OSX and WinXP, performance is pretty good anyhow. It's also important to remember that this is the beta of Vista.
And to you guys that think Windows can't cope with lots of apps running at the same time: I bet your definition of running 25 programs is loading them up and letting most of them sit their, idling. Loading up programs only consumes memory and as long as there is memory, the computer won't slow down. I regularly have about 20 apps running on my WinXP laptop and it, naturally, runs just fine. I've run my laptop like this without rebooting for over a month, as hard as that probably is to believe for most of you. It even went out of sleep without crashing with more than 20 apps open!
OSX probably handles high load from several programs better, if both systems are equipped with single core, non-SMT CPUs. It would seem that Windows runs very fast on systems with SMT (like the P4) or more than one CPU, though.