Optiplex 790 MT with a Plextor M6e PCIe SSD

My Optiplex 790 MT presently has 2 x HDD, 1 x SSD and the CD Rom, ie: no SATA ports available. I need to add another SSD and I am wondering which is the best option. Go with say the Plextor M6e PCIe SSD or a card that will give me more SATA slots.

My concern is that the M6e requires PCIe 2.0 x 2 and having read the manuals available I can not tell if this card will be properly supported my the Optiplex 790. Does the Optiplex 790 support PCIe 2.0 and would it install in the PCIe 4x slot ? 

Many thanks. Fin.