Your setup should handle Oblivion just fine...
I know this will sound silly. But some benchmarks show worse frame rates on lower settings with the 8800 GTX. (I think that's what you mean by Nvidia GTX 768 MB) Which kinda makes sense. Its an ultimate gaming card. So just as a test. Try running it at medium settings and resolution. And High settings and resolution.
Also,
this post has a very common reason for most lag and stuttering. And it has a fix too.
As for upgrading processors. Don't. You won't find much gain by an extra 0.6 Ghz. Honestly its alot cheaper to just overclock your current CPU to 3.0 Ghz. Which is the speed of the QX6850 (Wow I didn't even have to look that up! I am such a geek!

) especially since the QX6850 is over $1000 USD.