You are saying that your Power supply is 240watt? OK found the answer my self http://h18000.www1.hp.com/products/quickspecs/12543_na/...
Being a small form factor you will need a low profile card for it but since your PSU is only 240watts the best card I can recommend is HD5570 (beware though there have been some reports of compatibility issues with older motherboards). Another card with out issues would be HD4650.
No it does not! The HD5570 is very power efficient it has a little better performance than HD4670 and consumes less power than HD4650.
HD5750 would need external power from your PSU and probably needs about twice the power.