No point of getting the 3820 over the 2600k. First, I would suggest getting a better power supply and much better cooling. I seriously recommend you go custom liquid cooling on the FX-8150 because they OC like wolfdales (if you know what i mean ) You should be able to get a high overclock easily and you will notice a much better performance boost. As for the extra money, spend it on another 6970 for crossfire.
There is no reason to buy an i7-3820. You are better off saving some money and get an i5-2500k. Or you can wait for the Ivy Bridge socket 1155 to come out at the end of the month. Performance gains will be relatively small though in my opinion.
There is really no point in XFiring another HD 6970 on a 1920x1080 resolution monitor. However, it's your choice. You do not have to buy the exact same card for XFire. All you need is another HD 6970.