You can play whatever you want with that setup. Assassins Creed would run great(max settings) and is pretty fun, truck simulator I've never heard of and you can do better than airborne. You can pretty much max any game out there at a pretty high resolution. If you're looking for a story, I'd say Mass Effect. Fallout 3(I haven't played yet but hear great things) is another good choice. If you really want to see what your rig can do graphically, I'd try Crysis. It's a great shooter with an OK story, but it sets the bar graphically(and it sets it high!). Far Cry 2 looks great too, but lacks a story(still fun IMO). I vote Mass Effect though. Great story, great fun, good graphics.
As for quad core 2.4 vs core 2 3.0, I'd go with the quad. Not too many games fully utilize quad cores right now, but they will. And it's a nice cpu if you plan on overclocking. It's a good investment. The E8400 is nice too. But personally, I'd go with the quad.
I think you would be wasting money getting that DDR3 with a Q6600 unless you are going to clock it up to 3.0Ghz+.
Even still you don't see the gains from DDR3 until you get up to 1600Mhz and beyond and you need a i7 to really get the best from it. IMO you would be better going for a good DDR2 board and 4 gig of fast DDR2 1066.