HT1461: Mac Basics: Using Windows on your Mac with Boot CampLearn about Mac Basics: Using Windows on your Mac with Boot Camp
One or the other, not both. Both is only possible if you install Windows in a VM instead of BootCamp, but that method is very poor for gaming.
You can buy Windows 7 (Home Premium) for around $100
You can use Boot Camp to run BOTH OS X and Windows, but they can not be run at the same time. The cost of Windows 7 can be found by searching the web. Generally > $100. But that will be the only cost, as boot camp is a free utility that ONLY requires the windows disk.