normally on a OptiPlex 755 (and you need to confirm this by looking inside the case) the system has 4 slots for ram, ands 8GB is the highest in 99% of cases for 775 CPUs' thus DDR-800/667 being the two possible speeds used on the said motherboard.
Ao your looking at DDR2-6667 or DDR-800mhz, 4x2GB DDR2 ram NON ECC you will need to buy them as a quad pack or two sets of two
matching your current 2Gb to another is not a good idea on this type of motherboard, it will likely lead to more issues than solutions.
it will improves the system speed yes... I run a Q9400/8GB and when it was a 2Gb it was painful to use, now it runs win 10 and with a GT720 it is a perfectly good workstation to have in my office.
I would not use an SATA SSD, the loss of performance on a 6GB/s SATA they are designed force to run as a 3GB/s on your motherboard will not show great improvement over a 7200 HDD to be honest.