Correct!
The MB is very important for mining rigs - preferably go for an Extended ATX MB with 5-7 PCI-E buses.
The PCI-E bus speed does not matter. The CPU does not matter. Memory, also - does not matter.
What matters is the GPU(s) (and GDDR for Litecoin).

A quality MB, PSU and proper cooling is advisable though!
All the best.