[matm347]

I would say go with the dual core, games are coming out much quicker than I expected that support multi cores. The Quake 4 patch which adds dual core support made a big difference.
The 1900XTX is one nice card and would complement an overclocked AMD dual core nicely. IMO there's no such thing as overkill in the GPU department.

With air cooling, the case can make or break how much you can OC. I suggest searching for some reviews on the case(s) you like. You can have 10 case fans and still not flow, it's all in how well the case is designed.