For future Game go for Q6600 with a new cooler on it adn run it at 3.0Ghz or 3.4Ghz max.
E8400 vs E6850 both are fine go for the cheap one if you dont wont to OC.
Best Choice is Q6600 in my opinion if you can afford a cooler (1066fsb i so easy to OC to 1333Mhz.)
E6850 is not an extreme CPU, so is only OC by BUS setting I thing.
i have the same doubt about those CPU cos i am going to buy one.
And i choice E6850 (E8400 not supported) because I will buy it from a friend and i cant go for a GPU more powerfull than a Ati 4850 or a Asus EN9800GT Ultimate because of my cheap 600W PSU.