3040 uses Dell proprietary psu with 6 pin + 4 pin.

6 pinout is the same as 3050 and also 7080 or XE3

upgrade options: find Dell optiplex 7080 psu either 360w or 500w (the latter difficult to find now)

or find third party SFX psu with 24 pin and use 24 to 6 pin adapter

with your current 240w psu you may be already okay to run GTX 1050 etc. (gpu with 75w TDP).

GTX 1660, 960, 950 etc. need 360w+ psu if gpu requires one 6 pin PCIe power

higher power gpu that requires two 6 pin or 6+8 or 8+8 pin need 500w+ psu.