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.
I have been looking for a good PCI-e MB for a little while now, the best I can find (that is less than $200+) has 3, and that is on newegg. Where would you recommend shopping for computer hardware?