You've got a head scratcher.
1st point: both chips run most games at over 80 fps so you won’t notice the difference for most games so get the cheaper chip.
2nd point: Those links that Maizar gave are pretty useful, but what it shows is that different reviews come to different conclusions, some say duals are better, some say quads are better for gaming. For most current games the duals will win, just check out Toms charts.
Prey,
Quake IV,
Serious Sam 2 and
Warhammer Mark of Chaos run better on the E6750 and
Supreme Commander and
Unreal Tournament 2004 have higher FPS with the Q6600. You mentioned UT3, I have no idea which it runs better on, The Beta demo was released on Friday, so hopefully there will be benchmarks for it pretty soon and you'll be able to find out what it runs better on.
Personally I'd lean towards the quad because I do more than game, and the difference between 134 and 153fps isn't noticeable for games. There is a lot of talk about 'future games' that run better using 4 cores. Since I haven't seen 'future game' benchmarks all this speculation is just so much hot air, and I'll wait till 'future game' becomes 'current game' and I can reference a benchmark instead of passing on hearsay.
And now that I've gone and re-read your opening post, I've used that incredibly useful thing called GOOGLE, checked up on your mobo and it
DOES NOT SUPPORT QUAD CORE!!!!!
So unless you plan on upgrading your mobo as well, you only have the E6750 to choose from.
RTFM