Your manual has the answers- Dell Vostro 15–3568 Owner's Manual Best place to buy ram-- Memory RAM & SSD Upgrades | dell | vostro notebooks | Vostro 15 (3568) | Crucial.com Ram chips should match exactly for best performance. You would exceed the max ram in one slot with 12. Two 8gb chips would max out your model.
I wanted to upgrade my Pc Dell vostro 15 3568 to maximum ram supported and i wanted to know which ram to buy 2600mhz or 3200mhz ........Which one should I go for?            And please tell me whether it can support [12 + 4 ]GB  RAM combo or not???
Max ram supported per slot in Dell Vostro 15 3568