EDIT: I should have written:
"So the question is: Do I add 2X 4GB sticks to what's there and use it in two channel mode, or do I just scrap it all and buy two kits from Crucial...a 12GB and a 6GB and use it in tri-channel mode?"
Also, I noticed the Dell RAM is 1333MHz, while Crucial only has 10600MHz or 12800MHz. Will the speed difference be significant...again, for Photoshop work, not gaming? My goal is to have 2-3 programs such as Adobe Bridge and LightRoom open while working on 500~800MB photos in Photoshop without slowdowns.