Dell's maximum supported RAM for this system is 16 GB. However, it works fine with 32 GB of RAM. I used this memory kit in mine:
Corsair XMS3 32GB (4x8GB) DDR3 1333 MHz (PC3 10666) Desktop Memory (CMX32GX3M4A1333C9)
Probably other ones will work fine, too. The only caveat is that most Dells would have come with Windows 7 Home Premium installed and that will only allow access to 16 GB of RAM. You need to have Windows 7 Professional or Ultimate to access the full 32 GB of RAM. You can do the Windows Anytime Upgrade to buy Win7 Pro if you want to go that route, basically it magically makes your existing install into Win7 Pro so you don't have to reinstall anything.
If you put more than 16 GB in a Home Premium system, when you right-click on Computer and go to Properties, you'll see the memory listed as something like 32 GB (16 GB usable). So the OS will see it all, it just won't let you USE it all. Harsh, ya?
I had looked this question up and not gotten anything solid for an answer so I tried it myself and am posting about my success.