without knowing what your intentions(specific games, resolution, in-game settings, etc) for this card may be, no one can offer any "upgrade" advice.
but being limited to a 450w power source there really isn't much viability to be offered at all.
you would need to include your budget,
what PCIe power cables are needed(if any),
and what space you have to fit any card that just might be suited for this situation.
a much better option would be to move away from this prebuilt junk and start a custom build in a new platform.