Max 8300 can support is 16=4x4 gb ddr3. your 6gb may be two sticks 4+2 or three sticks 3x2. not sure speed.  Is it 10600 or 12800? if 4+2, can buy another 4+2 of same speed matching the original. cleanest upgrade though is to get 4x4 gb identical sticks.
 I have 6 gb Memory dell xps 8300 want to increase memory should I buy another 6GB or 16 GB that won't cause any conflict. thanks
XPS 8300, memory upgrade from 6GB