What you need to upgrade to the gts250 is a pci-e slot. A quick look on the internet did not provide a clear answer. The best may be to open up the case and look.
You are looking for a slot similar to the long yellow one in the above picture.
Next you would want to measure the amount of room you have from front to back where the card would go to ensure that the card would fit. Luckily the gts250 is shorted than in predecessor the 9800 so you may be in luck.
Finally you want to see what type of psu you have to ensure that you purchase a psu that will fit inside your case.