Nope, well It would work better with a Quad Core and DDR 3 ram of course but Nah you wont get a major bottleneck. I think you should go for that GPU
Why would it work better with DDR3?
If you recommend DDR3 then OP will need a new Mobo.
Depends on resolution, at low resolutions with lower details the CPU will bottleneck, at higher ones there won't be much of a noticeable difference.