Buying a half-height gpu for an Optiplex doesnt make a ton of sense with the $100 budget - isn't financially feasible really, least nothing worthwhile out there. I saw a 1050 half-height for like $150 usd.
What is it that you are trying to achieve with the machine as far as 'better performance'?
What are the specs of the version of the Optiplex 980 that you have?