If you want to run games earlier than 2010, then you may as well just install Windows 7 or Windows XP. This may also rule out some incompatibilities since its closer to the original OS the games were designed for. I know you didn't ask about running older windows OSes, but its a suggestion. I'm running Windows 7 via bootcamp on my MBP 13" mid/late 2012, so you shouldn't have a problem. I'm sure Windows XP works just as well.
I have never used VirtualBox, and I've heard a lot of good things about it. But I'd just say bootcamp it. While having to restart to get into bootcamp is kind of annoying, getting full speed out of your computer with really no 'tricking' is nice (especially if you are playing games. Restart to play games. Restart again back into OS X when its time to get work done. Simple). It won't damage your OS X install (if you're careful and read directions, at least). You install windows with bootcamp, which is from Apple, and they don't really want you to damage your OS X install anymore than you do.
As for DOS games, I second the vote for Boxer. I have used Boxer before, and its a good application.