Well. let me ask you this.. Is there anything you do with your computer now that you think could be done better? If the answer is no, then I'd wait until such a date and time when the answer is yes.
I went back and read your original post, it doesn't seem you have any complaints about your current setup, rather you're concerned about a future issue that may or may not materialize. The 6950 is a decent video card, if thats the one you have now, but there are better ones out there.
But yea, your overclock is decent. Maybe you could get a better one out of a better board, I just don't think its worth it. You're only guarenteed the stock speed when you buy a CPU, overclock-able model or not. Overclocking is just gravy. The turbo only charges up when its called into service, so I wouldn't look at it as only being 200mhz over the turbo, its 1GHZ over the stock speed. You're running all 8 cores at that frequency constantly. And yes, 4.2 is a nice OC on any CPU.