People are going to tell you to spend the extra money and get the i5 2500k which isn't a bad idea, however I would first upgrade your mobo as the i5 2500 + HD 6870 is more than enough to play any game on ultra settings probably for the next 2 years.
...i5 2400 + HD 6870 is more than enough to play any game on ultra settings probably for the next 2 years.
Are you joking?
I have the 2500k clocked at 4.20GHz with two 6970s crossfired, and I just now had trouble running Crysis Warhead (last level) with all the Aliens and action everywhere. I was getting sub 50 drops. Of course, I have some texture mods, but still...
And that is just one game. There are a lot of demanding games out, with more on the way. There is no way that a single 6870 is going to handle every game on Ultra for the next two years. It may very well handle every game, and maybe some (or most, even) at high graphical settings with a reasonable resolution. But it will in no way max every game, or even close to every game, for the next two years. At least not at playable frame rates.
$10 for the 2500K and $30 to bump to a Gigabyte Z68A-D3H-B3 for future Xfire leave you at $750. After that, I'd like to see a nicer graphics card and maybe a slightly more powerful PSU (for Xfire/SLI), but it's up to you how much you want to spend.