Since you do not have the PCI-E power connector, I would suggest the HD 7750. The only exception is that if you do use the Molex to PCI-E adapter that usually comes with the cards, make sure it's connected to it's own rail. This means you cannot have another else connected on that rail (Single cord with all the Moles connectors.) as the GPU will need that rail for it to work.
This is the Adapter I am talking about that usually comes with the cards.
This is what I mean that it should use it's own rail line. make sure nothing is connected to this cable, you can connect fans, that's it, nothing else, use another Molex line cable for your HDD, DVD/CD drives.