For the fitments, single slot, low profile graphics cards will fit.  If your variant of GTX 750 is double slot card, it will not.

For power requirement, a 250w power supply is suggested.  Although OP did not disclosed the PSU info, the OptiPlex 3040 SFF has 180w power supply.  It should be taken into concern and considering for upgrade.

even though the card has a single slot bracket, it may have a heatsink > 1 slot width thick, such as 1.5 slot width.  the 3000 series SFF does not support card with thick heatsink due to psu only 1 slot width from PCIex16 slot.  if you move the psu outside case then it would be possible.