You can do O.K for 800, but you won't exactly be... Impressed..
If you're hell bent on spending 800, maybe read through this:http://www.tomshardware.com/reviews/value-gaming-pc,2578.html
Maybe try and go off this? But wardy22 is right, you need an OS, and the cheapest* is Windows 7 Home Premium OEM.
*Linux is free... But don't push it :P