Yeah, your BIOS on your Motherboard needs to be updated in order to use the full dual core aspect. A simple BIOS flash or a motherboard with dual core ready BIOS is all you need.
Go with the 4200+ and 7800GTX if you do not plan on upgrading later then. In graphics, the 7800GTX outperforms two (2) 6800GTs.
Also, might I recommend a different motherboard? The MSI Neo4 Ultra is good, but is a LOUSY OCer!! HORRIBLE!! Check out these two boards and dare to compare them! I think you will be pleased!!! This will allow you to take the 4200+ to 4800+ levels with ease.
This is DFIs pride and joy. A good OCer, but tedious to get running in most cases. It will require a BIOS flash.
This is ABITs pride and joy. This is the same motherboard I use, except not dual graphics cards (SLi). The FATALTIY is easy to get setup and a good OCer as well. Its cooling is excellent as it uses silent OTES technology on the board. I recommend this one for top performance. NO BIOS flash is required, it is dual core ready!