Need advice on new computer to buy

I'm in Canada and looking for a new desktop. I can build my own or I can order a pre-built one, basically have a price range around $400-900 and I'm looking for the best deal around now, basically just want to play the total war games on it so it will need a decent enough video card and enough ram

Do you need an OS?

if not then you can do this type of build for $862

i5 4670

Z87 motherboard

8gb DDR3

1tb hdd

550w psu

HAF 912 case

dvd burner

GTX 660 or R270x

If you need an OS

Change the cpu and motherboard

FX 6300 and a 970 based AM3+ motherboard

http://pcpartpicker.com/p/2wOit

I think this is quite good.