According to the Dell manual, specs, 8GB is maximum.

From the Dell Manual:

Memory Type DDR3 Speed 1333 MHz Connectors two DIMM slots Capacity 1 GB, 2 GB, and 4 GB Minimum Memory 1 GB Maximum memory 8 GB

8 GB maximum for one slot or two

And i want to tell you that my Ram speed is 1600 MZ

NOT 1333

Look at the Spec's I posted, 1, 2, 4 GB per slot.  For best operation you need two (same) memory modules for dual channel (faster) operation.  If you have higher speed memory (and it will work in the PC) it will only run at the maximum speed of the memory bus, which from the Dell published specs is 1333Mhz

16GB will work just fine in this machine. And since you have an Ivy Bridge CPU, 1600Mhz is default for memory.

If you want 1600mhz, you have to buy RAM without XMP since XMP is not supported by the BIOS.

I can assure you that two 8GB Kingston Value RAM (KVR16N11/8) will work just fine with 1600Mhz.

I tested the memory for more than two weeks now, did a few memtest passes without errors, ran prime95 blend test for a few hours, without any errors. I've been having no bluescreens or whatsoever... So I can consider this combination to be stable. Also make sure to update to the latest bios. I've been using A21 since I have this machine.

What RAM can you recommend?

I can only say that Non-XMP RAM will work for more than 1333Mhz

2x Corsair 8GB XMS3 1600MHz work fine have them on my machine for 6 months no problem at all.

Dell, in many cases, does custom implementation of the Intel chipset, thus the Intel specs do not always apply.

For example, I had an old Dimension E510 (5150) and the Intel specs for the chipset in that model listed a D965 CPU as being compatible (D820 through D965 CPUS) however due to the Dell implementation a D945 was the best that could be used.  We had several users try a D965 and it would not boot with the D965.  Also max memory was an issue.  Max in this model was 4GB per Dell specs.  Users tried more and it wouldn't work.