Right where I suggested, I found the OptiPlex 9010/7010 Small Form Factor Owner's Manual that includes a Specifications section. It states the following: RAM: Four slots, DDR3 1600 MHz, up to 16 GB total Drives: One x 3.5-inch SATA drive bay + two x 2.5-inch SATA drive bays Crucial says that system can run up to 32 GB, so for that spec I would go with Crucial since they test systems beyond Dell's original specs. Regarding drives, you need to buy SATA. That PCIe slot is not intended for drives, so speed would be less. Any SATA SSD you buy now would be SATA III as standard. There are several listed on the Crucial page that I linked for your system, though you don't have to buy Crucial.
Has anyone changed the HDD to an SSD? if so did you use sata or can you use a PCI-e slot and have it bootable? I have searched this Dell site and can not find any specs on this motherboard.
SSD and Memory Upgrades