the 750 ti was made for just such an upgrade. though not as strong as a 270x it will still give you a HUGE boost over the hd 2500. just check the specs for a model that does not have a 6 pin connector (there are plenty of these models) and you'll be good to go.
of course this is assuming you don't have a 6 pin connector on that psu. it might but i doubt it. if it does, then you can pick any 750 ti you wish as it will still barely draw more than the 75w the mobo gives it