CPUs and a board are two different things. Your happens to be a S775 Core 2 Duo E6400.
As for you new video card not working, what does it do actually? Your board (ASUS P5N-E SLI) has PCI-E and if you have bought a card that is PCI-E then I don't see why it won't work with it.
As for you new video card not working, what does it do actually? Your board (ASUS P5N-E SLI) has PCI-E and if you have bought a card that is PCI-E then I don't see why it won't work with it.