Can Litecoin mining be bottle-necked by motherboard/cpu? So I brought 2x Sapphire Vapore-X R9 280X video card.  I am only doing 550kh/s each while on my other more powerful setup with the same card get about 750kh/s.  I am wondering if the motherboard/cpu matters?  Here is what I got on my new rig:

Asus P5K

Intel Core 2 Duo @ 2.66Ghz

2GB DDR2 Memory

As far as my research goes, this should all be fine, but at the end, the hash rate does not lie, what am I doing wrong?
I highly doubt it. Even the PCI-e connectors do not really matter. I've got less CPU power and RAM than you and my rig is controlling 4x 7950s at full potential. The CPU/Motherboard is essentially calling functions. The functions themselves are what cost so much, and those are computed by the GPUs.