Do you want to spend time making your own computer or just have a computer you can plug in and run?
One benefit of a prebuilt computer is that if you run into problems the warranty terms are better. If you build your own the warranty terms will be down to just the certain component that is acting up vs telling a brand that they need to send a replacement computer.
Building a computer is pretty easy but comes with risk and takes time and one bad move and you can destroy parts.
Since your looking to spend $1200 Canadian to get the best/most for your money... I just need a direction you want to go... do you want to be bothered and take on the risks associated with building your own computer or do you just want to open a box and boot it up?
I use to play on line games but stop playing long time ago but I like role playing games and war games and dungeon and dragons
Also a list of game titles that your going to play will be a help?
Games resource requirements depend on the games. Example being Facebook Games are mostly Flash Based execution and Flash Games will generally only use 1 core of a multiple core system, so a Dual-Core 3.0ghz low cost computer could run a game like this just about the same performance of a 8-Core 3.0Ghz system.
Computers are evolving pretty quickly and you could go with a $500 computer now and then another $500 computer 5 years from now and stick with a 5 year life cycle or maybe even get 10 years out of each one if games are simple flash based etc. But if you will play any games that are graphically and computationally complex then a light weight computer would not be well suited for you.
As far as OS goes. I personally like Windows 7 the most, however Windows 8 or 10 has more of a life for microsoft support. I am guessing you already have a computer now... what OS have you been using that your use to and have you worked with Windows 8, 8.1 , or 10 to know whether you would like it or not?