Check this out. This will allow you to run all current titles at ultra settings/1080p at comfortable frame rates. It comes in at around $1000. You said you needed an OS...that'll be $100, unless you know what you're doing. It can be obtained other ways...
Then you said you'll need a monitor. You'll want one with at least 1080p res... that'll be $150 or more. This should all come in right at your budget...maybe a little more or little less depending on day of purchase. If price drops occur and it fits the budget within a few months, I'd go with this build...http://pcpartpicker.com/p/c4xG
It's a personal build I've been researching for a while now. It looks good, and it's a strong performer. It's a little over your budget if you have to buy an OS and a monitor, but it'd definitely be worth the extra cash. Hope this helps