I never said it would remove the bottleneck. I said it would check for a bottleneck on the GPU by making a decent amount of improvement. I expect it would scale fairly well.
I do agree that the chip would have to be pushed pretty high to remove the bottleneck on the X1950, no argument there. Around 4GHz is a decent estimate, I'd say.
I did tests in WoW With the 3800+ X2 and X1900XT. I'd hang out in front of the auction house in Ironforge, on average the most crowded place in the game. I tested the difference between the stock 2.0GHz and an overclocked speed of 2.1GHz. There was a big difference in frame rate when overclocked - much smoother. I also did tests in FEAR, saw a ~5fps difference in favor of the overclock.
Overclocking is also relative. Robs says it took 2.6GHz to remove his bottleneck. I don't have huge loads of dispensable income, so I didn't overclock that high. But it didn't take much to see that there was a slight bottleneck.
You may want to think about upgrading to a C2D. You have the board for it, so you might as well spend the ~$200 and get an e6300. That will pair nicely with your video card.