You can't just think of it in terms of GHz fishie
Also, a bottleneck is basically whatever part of your computer that is "holding the rest of your computer back" and is usually the part that will yield the best performance gains when upgraded. Say you have a X850XT paired with a Athlon XP 2500, would your gaming performance improve more by upgrading the video card or the CPU? Obviously that CPU is already bottlenecking that graphics card so getting a faster one is not what you want to do.
_I need to point this out by the way, even if you are CPU bottlenecked, say, 3000+ Venice and 7800GT and you upgrade that GT to a GTX, you WILL get better FPS and that doesn't mean you are not bottlenecked, it just means that neither that GT or that GTX will be working at their full potential until you get a faster CPU_
the reason for this is because your graphics card is not the only thing doing graphical work, your CPU actually has to send data to the video card before the video card can draw the next frame, _now I am going to start talking mathematically, and yes I do know that the data sent between a CPU and GPU is not measured in FPS LOL!_ let's say your CPU is sending data to your graphics card at 50FPS but your graphics card can process each frame at 1/70th of a second, that should mean you get 70FPS but your CPU is only sending 50 frames to the GPU each second so in the end you get 50FPS, now upgrading that GPU to one that can process 100FPS in the same conditions is not going to help you out nearly as much as getting your CPU up to your GPUs speed.............
I hope SOMEONE understood that, that was SEVEREALY dumbed down. lol.
Oh, and yes you are CPU bottlenecked, your 7800GT should be easily able to do 8300-8500 if you had a CPU that was totally able to keep up with it, unfortunately, I doubt even the new 3GHz FX59 will be up to that task, the best thing you can do is just be happy with what you got and start enjoying your games