Depends on what you mean, by bottleneck. Are you referring to gaming performance? I assume you are.
Also it is important which games you play, at what resolution etc.
Give us some more info so we can help you.
Why do you think you have a bottleneck? What are you running that isn't flowing smoothly? What settings?
Quake 4 benefits from from a Dual Core CPU (one of the only games)
Oblivion is bottlenecked by nearly any graphics card.
I really can't help you much unless I know more...
But to answer your question
Is P-4 3Ghz a bottleneck 4 7900GS?
In some situations, yes. In others, no.
Gaming at lower resolutions, with lower AA+AF settings, you might be missing out on high frame rates because of your CPU, you would be getting medium FPS. For example, in Doom 3 you would probably be getting around 85 FPS with that CPU. If that is enough for you, good. If it isn't, a better CPU will get you higher scores.
Take a look here:
http://www23.tomshardware.com/cpu.html
A 3.0GHz P4 is getting 88FPS in Quake 4, the E6400 (2.13GHz) is getting 159FPS.
Serious Sam 2: 61 FPS vs 102 FPS with the mentioned CPUs.
I wont go on. Take a look for yourself.
But 60+ FPS in a game, especially 80+FPS is still really good. Depends what you want...
The frame rates above are with another faster graphics card. But the Doom 3 frame rate is with a similar card.