Buy whichever OS you find cheapest, though I suggest 8/8.1. Will probably cost around $100 which leaves you with $800. Buy another monitor, probably costing around $150ish. Do NOT run the game on both screens. Your crosshair will be in the middle where the bezels are and will make gaming very hard. Use the second monitor for music, browser, chat, etc. Your now down to $650 or so. I'm guessing this puts you in the 6300 and 270 range.