I'm sure he doesn't care about bottle necking and he knows its already 3 years old...
This is the official spec of that card he currently has "1600 stream processors and 230GB/sec of memory bandwidth provide the brute power "
I assume he wants to run DX 11 Titles only (not caring about the bottlenecking comments)
I would recommend a 6950 (1408 streaming processors on board) or a 7950( 1700 streaming processors)
Both have large amounts of on board ram, the 6950 is obviously cheaper but the best bang for the buck.
Both cards will be more efficient and offer similar performance with your CPU.
I don't care to listen to these bottle necking comments. If the game runs comfortable enough at 45-60 FPS on high and in some cases maxed ultra settings in 1080p, good enough. If you want them to run faster that that, definitely get a new CPU.
I use a 6950 with a Q9550 and it does the trick with no slow downs in any game currently.
Either of the 2 cards would be a good upgrade to DX 11
I doubt you're limited. Even if it bottlenecks a little bit, it will not make your PC work any worse. Just that GPU usage will not be at 100% all the time. And it will only happen on extremely CPU oriented games. Also, if you decide to upgrade your CPU in 2 years or so, you'll still have a very very very capable GPU. I still stay with my recommendation of GTX 670.
I suggest a Radeon 7950, but like the others are saying, your CPU isn't stellar. However, it shouldn't be a very big issue. I currently run a 6870 with a 2.3 quadcore, and it runs just fine. So if you're buying this with gaming in mind, I highly suggest the Radeon. However, that's just because I've always used and loved the Radeon series. Do take a look at the PSU requirements on whatever you choose before buying, though. I had to upgrade my PSU after blatantly overlooking it when I purchased my 6870. Good luck!