If your budget doesn't include a monitor, then I think the best you can do is what ismaeljrp gave you, except if you get your processor and board from microcenter (their ivy bridges are significantly cheaper, like $50 and they have a promo that gives u an extra $50 off if you buy a matching motherboard), you might be able to pull off a GTX680 and then you'll be a beast.
Sadly, my budget is around yours as well except i need a monitor
EDIT: ismaeljrp, do you think that power supply has enough juice to run that rig + overclocks from both cpu and 680?