Hello .. Im building a new pc for work ..

Im buying this motherboard  Gigabyte GA-Z77X-D3H  , I already have 2 sticks of 4 GB  each one  they are:

Corsair       CMZ8GX3M2A1600C9 8GB 2x4GB DDR3 Vengeance 1600MHZ 1.5V XMP RAM CAS9

and I want ( or at least my idea is ..)  to install  other 2 sticks of 8 GB each one

Corsair    CMZ16GX3M2A1600C10 16GB 2x8GB DDR3 Vengeance 1600MHZ 1.5V XMP RAM CAS10

both  memorys are    1600 mhz , both are :1.5V the only difference is that  one is ¨CAS9¨ and the other ¨CAS10¨

my idea is to put them this way ,in order to make them work in dual channel (?) : 4-8-4-8  =24gb of ram ?

my doubt  mixing  4 and 8 gb sticks .. I would get  dual channel ? or is  not good,,its a waste of money,  and better get another  couple of 4gb  to make 4-4-4-4  ..  ?  any help would be very  appreciated!

thanks in advance