As far as gaming goes, I used to play CS 1.6 through WINE. It wasn't as good as on Windows, but at least for starting out I would reccomend dual-booting.
You don't have to clean your hard drive. If you have enough free-space, you can resize the one big partition into a total of three (minimum you would need for Windows and Linux to play nicely.)
There are multiple products out there that will allow you do that, but it isn't the safest way to get things done. It works well the majority of the time though.