Depends on the Optiplex 755, it comes in 4 sizes. If it is a small form factor machine or a slim case, then a new case would be advisable as you would be limited to low-profile low-power GPUs.
If you don't mind smaller less powerful GPUs, than it should be okay. Though trusting a power supply that old with a heavier load is a little risky.
But really need a total budget to make a decision.