1.
Check if the new card is a PCI Express slot. It will say so on your motherboard, right next to the slot for your card. It says something like PCI e x16 or something. I don't rightly remember. (this is also assuming the gts 250 is a pci express card, which it most likely is, because everything before it is probably obselete now) If so, you can put the graphics card there. Also make sure your computer case is big enough!
2.
Check the power supply requirements on the new graphics card.
See what yours is.
Finish!