Those 250W Inspiron supplies wont give you much to work on as far as connectors, it also lacks the output for a card that draws 106W like a 450 GT. The PCIe also will only deliver a max of 35W which leaves out a 750 and 750TI so that leaves you with a small amount of upgrades.
720 GT No additional connector 19W... pretty weak for gaming
730 GT No additional connector 25W.. Still weak for gaming but not much else out there.