Venice w/ a 4000+ Venice on the Doom III 1280 Frame Rate comparison. 79.70 vs 80.70 -- a small difference you could never see with the "naked eye".
On Wolfenstein the difference is larger - 21% (157 vs 190) -- still I'm not sure the difference would be noticeable while playing the game.
I just checked several other benchmarks and the difference between the two is small enough that I don't think you'd really notice it. Last, I checked the price performance benchmark, and the 3000+ venice blows it away.
Of course their is the "ego" factor. If having to tell your geeky friends that you bought a 3000+ will bother you (cause they all have at least 3800 or dual cores) then maybe you should go a little bigger

And there is an old saying that goes something like "You will have to live with this purchase decision long after the joy of having saved a few bucks wears off".
So far I'm still happy with mine - hopefully I'll get my joystick issue solved.