There were a lot of different Optiplexes. But from Core2Duo on up they have 2.0, or 2.1 sometimes originally, sometimes upgraded with a BIOS update. 3.0 is not needed for any GPU you can put in an Optiplex. maybe not for any GPU period. A few of the smaller models had a 37W 16xPCIe slot. But mostlt they're normal.
The space available around the slot is another matter. 2 slot cards tend to hit things.