So I am planing on buying new computer (Alienware x51 Extreme Version) And I can decide what will I buy Windows 8 or Windows 7 I will use this computer for gaming...(Both are free) Thank you for your time
That's only free mainstream support. They are not ending support altogether for Windows 7 and will continue to provide patches and updates until 2020 (this will likely be extended just as with XP). Many businesses have moved to Windows 7 from XP only within the last few years and updates for XP ended only within the last year. Windows 7 support will continue to be available and if you're buying a PC through Alienware, they will provide support for you according to your purchase agreement.
Windows 8 is definitely getting better, but I think Windows 7 is still the best bet for now.