Video editing is CPU intensive and so the Q6600 is the better CPU. If your not overclocking, there won't be a noticeable difference with the higher case latency, really comes down to your budget. Other than what's in the BIOS I don't overclock so will leave that to someone else. If you have a camcorder with firewire, I'd recommend the ASUS P5K-E, has 3 PCI slots as well. A good less expensive board without firewire and only 2 PCI slots is the Gigabyte P35DS3L
1. Yep, you're correct the E6850's 3ghz will beat out the 2.4ghz quad. It's good for gaming. Quad will beat out dual in video/audio editing. If you game more than work, get the dual. & vice versa.
2. Makes absolutely no difference for a stock pc.
3. Yep, it's a no brainer if you have the right tools. Why not? It's not a workstation, downtime means clearing cmos in minutes. but if you do o/c, get a good aftermarket cpu cooler & maybe upgrade the case fans to Scythe high-cfm. 60-90. & a 4-fan fan controller.