I run the same resolution. It's around 80% more pixels than 1080p. So you'd effectively need that much more power to run it at the same settings at the same frame rate. (Perhaps not perfectly scaled like that.. But it can't be too far off).
Few hints; At this resolution; on a 27 inch screen, the pixel density is quite high. You don't need AA/MSAA/FXAA at all. Trust me lol.
The HD 7870 2GB would be the MINIMUM you'd want to upgrade to. But frankly to run BF3 at ultra at this res; you're looking at a HD 7950/70 honestly. If you can stand everything on High; you'd be sweet with a HD 7870 (which is very similar to GTX 580 performance). Oc's better and uses less power too.
But; xfiring is NOT the way to go. BF3 maxes out 1gb VRAM at 1080p. How do you think two cards with still 1GB vram will go at 2560x1440? =). Absolutely horrible lol.
I'd go a minimum 7870 2GB; but honestly you're looking at the big boys for this resolution.. I had HD 5850; changed to the 580 specifically for this monitor. (580 was the fastest single card out at the time). And the HD 5850 is right behind the 6870.. So.. ya. You'll need to make a little bigger jump than that my friend =(.
HOWEVERRRR. I'm talking about a heavy map. Or MP. Single player; you should be fine with 7870 or 7850.. The 7850 performs short of a 6970 by default; though once OC'd you'll find it'll beat out a 6970 and a GTX 570.. 7870 is like 1% behind a GTX 580; and has 500MB more VRAM. And OC's very well..