I can't see what that's plugged into. Is it plugged in to a port the size of the port below it?
The reason I ask is that those are PCI-E x1 ports and the 6450 is the best card I can find that fits into an x1 slot.
However, if the slot itself is about 4x the size (a full size x16 port), you should be able to do any video card that doesn't need an outside power connector with your power supply. That would probably be a Radeon HD 7750.