Dell T320 want to add memory, but can I add only 1 module or a pair?

I've got a Dell T320 with 16GB (4x4).  I've got a total of 6 slots.  I can live with 24GB but can i add a single 8gb module (remember, the first two pairs are 4gb or do I have to add 2 8gb modules and have 32GB?

Thanks,

Roveer