You probably won't find a new radeon 6950 that can be turned into a 6970 because AMD changed the way they lock the cores from removing them in the BIOS to lasering them. Also, why would you get an AM3 build if you will be getting an Ivy system that comes out next year? I hope you also know that there won't be many Ivy board that supports 32 PCIe lanes for dual x16, they will basically have the same connectivity as sandy-bridge. Also know that unlocking an AMD CPU's locked cores fails more often than it works, I've read it has about a 40% chance of working.
Also know that you won't be able to overclock the CPU as much as you could with four cores as you can with three cores, suggesting your successful.
To be honest, I suggest you wait until Ivy comes out before you buy anything. It probably won't be much faster than Sandy but it can drive prices down.
You shouldn't mix RAM kits if you can avoid it, period. If you do they must be logically identical to get the highest chance of success. When it comes to overclocking your RAM significantly I don't just recommend that you don't mix, I tell you your chances of success are slim at best. If you want a 2000MHz+ overclock then you should aim for 2133MHz. You can either get a 2133MHz kit, a slightly slower kit and overclock it, or a low voltage kit and overclock it. The cheapest option right now would be to go for a 1.25v kit and overclock the hell out of it but that is also the most likely to be problematic.