My apologies. I did not see the CPU model number when I first looked at the screenshot. According to the
Intel spec sheet the fastest memory your CPU officially supports is PC3-8500 DDR3-1066 (1066Mhz). This is also confirmed by the Dell setup manual for your machine. You can often run much faster RAM than the official specs on enthusiast boards but OEM's are a lot less forgiving. You most likely went with an OEM machine because stability and reliability are your #1 concerns so I would not buy RAM faster than 1066Mhz. Buying faster RAM
should still work though as it should just slow down to 1066Mhz.
24GB is the limit and if you divide that by 6 you get 4GB DIMMs. The Dell manual does state it supports "capacities greater than 4GB" but that's a bit ambiguous - I would stick to 4GB.
So for optimal use and trouble-free operation you want a triple channel kits, 3x4GB 1066Mhz 1.5v inserted into the correct slots to maintain the tri-channel configuration but your current setup is 2x4GB and you could buy another 2x4GB kit and run two dual channels instead if you wanted to reuse this RAM.
Dell service manual
Dell setup manual
Both of these have information about your memory options.