In order of performance (low to high) are the 5450, 6450, GT 440, 7750 (800Mhz version)
Also, you may need to look at your motherboard and see if you have a PCI-E x16 slot.
All of these don't need a power supply connected to the cards. All of them are capable of working in your computer with a 250W.
If you don't have much room, go for the low profile versions like this:
You can see that the top half of the card is cut offhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814127571
Let me know if you have any other questions