Of course you can use a WD Green as a boot drive. I have had 4 WD Green HDDs in my i5 Win 7 64-bit PC for the last 4 years. One of them obviously is the boot disk. I've just set up another PC for my son (Intel Quad Core 9400) with Win 7 64-bit on a WD Green HDD. I plan on doing the same when I re-set up my wife's computer next week. There's nothing wrong at all with WD Green HDDs.
While the Blue HDDs are good, the milliseconds you save aren't practically useful.