Yeah an i7 920 at 3.8GHz is still quite potent. It can definitely still keep up with your GTX 670 without any trouble. Unless you have some specific CPU heavy application that you really want to run faster, and that application can effectively use 8+ threads, there isn't much sense in dropping lots of money to upgrade to Sandy Bridge E now.
I'd say wait and see what comes out next year, and then maybe upgrade then if it is significantly faster than what you have now.
I'm guessing your performance is still flawless (as it should be with that setup).
Nehalem @ 3.8ghz is still awesome performance.
I'm still using a 4ghz Q9550 + 6950 w/ unlocked shaders and loving it. Even I'm waiting. Graphical innovation has halted since Crysis(1) and I don't see it growing too much even with the next generation of consoles. Thank console gamers and developers. Also, it sadly looks like next gen consoles will be using 6670/7670 GPUs so (most) developers won't be pushing the graphics envelope like they should and use to be.
If you were looking at adding another couple 670s to your GPU setup now/soon then I'd say go for it, but for now definitely wait for haswell/skylake.