You're better off keeping your operating system on the slower drive, and keeping the fast drive purely for games. 150gigs may seem to be a lot, but after you've installed 15 or so games, you realize that you could use some more space, especially since newer games want anywhere from 1-5 gigs to install. There's no reason for your OS to waste space on your fast/expensive hard drive, when you won't notice a difference if it's on your slower drive.
Keeping everything else on your larger/slower drive is a good idea, because they don't need the performance boost that playing a game would demand. I would keep the 10k rpm drive solely for games, and use the 500gig for everyday programs, files, music and videos, whatever doesn't merit a performance need.