Question about HP ProDesk 400 G3 MT

I can't imagine that the power supply has much more power than the PC needs. But you can certainly install a power-saving graphics card (like the Gtx 750ti).

I have installed the GeForce GTX 750 Ti in a colleague's computer. It works perfectly so far.
He even plays Anno 2205 with it, so it's certainly sufficient for office use.

My question tends in the similar direction: I want to attach two 24'' monitors to it, which graphics card is recommended? (Office application only)