GeForce GT 710
Evan: The Optiplex 990 MT can support longer than half length but not full length PCI-e cards. According to the technical guide book it is limited to 7.4 inch (24.13 cm). This assume you have the MT (mini tower model). The DT and SFF models are small form factor designs and limited to half height cards. The best of which is currently the GTX 750 Ti 2GB.
You would want to stick with one of the mini (single fan) models of the GTX 1060 6GB
This EVGA is the right length
So is this Gigabyte
As the Power Supply Unit (PSU) is only 265 Watt. You will need a better unit. As power supplies are important for delivering clean safe power to your components. I'd recommend at least a decent unit from a reliable manufacturer. As they have a fairly long use life. It is easier to justify the cost. You can always use it in your next computer.
This Corsair BQ 650W provides plenty of power and has modular cabling for a cleaner installation. If you want to save money they also have a non-modular 650W 80+ Bronze model. 80+ Bronze is a certification where a PSU is at least 82% efficient at the stated watt rating of the unit. Personally I would buy an 80+ Gold unit from EVGA or Corsair. They are more efficient and some have a 10 year warranty rather than 5 year. But I did not want to shock you with the price. While more power than you need the 650W unit has enough headroom if in the future you ever want to use a high draw CPU and GPU.
Need to upgrade power supply for sure
no you can not for one the power supply is 2 small
I don't think that case will fit it well. If you can use a standard mid-ATX case; you'll be fine.
If you got money upfront yes