I would like to upgrade my SFF Optiplex 390 with a better GPU.
Would a 750Ti work or do i have to look for another card, if so, which one?
Is the 240W PSU enough to power it?
And would it make everything a lot easier if i just buy a new case and PSU?
No, that system isn't going to take a 750Ti. And replacing the case while retaining the proprietary mainboard isn't a trivial exercise -- essentially, you're going to be building what amounts to a new system.