Any graphics card made in the last few years will need more than 250.
Get yourself a 300 and one of these:
http://www.newegg.com/Product/Produc...82E16814487025
Or something similar. The nice thing about the gtx 750 is that it doesn't require two card slots or extra power, and has an extremely low power consumption while still providing excellent performance for a $150 card.
I'm in a similar situation and have just been too lazy and haven't needed to upgrade my power unit. As soon as I do I'm buying one of those (the built in HD 4400 on the i3 processor is surprisingly powerful - it won't run things at top specs, obviously, but it runs even the newest games at 30+ fps on medium settings).