Personally, I'd go for the 6850 or the GTX 460 (whichever's cheaper), as both will rock at your current resolution, and consider a Q6600- as Fallout: New Vegas is a console port, it will use multiple cores, as the Xbox 360 has 3, so you'll get significant gains from the upgrade. Don't plan for CF- your PSU won't be able to cope with two powerful cards, although it should be fine with one 
And remember- you might be able to sell off your E6300 to make some of your cash back.

And remember- you might be able to sell off your E6300 to make some of your cash back.