I replaced my ATI 5570 graphics card with an ATI Sapphire 6950, what power supply should I use?

Just recently purchased this HP Pavilion p6621c desktop. I replaced the stock card (5570) with a Sapphire HD 6950. To my dismay I realized the PSU of this computer is a measly ATX-250! As sad as that is, everything seems fine so far but I don't really game much. I don't want to run into problems in the future so I'm wondering what PSU I should buy to replace it.