what makes a graphics card compatible or not.
Se couber no slot, é compatível.
Power supply wise, the new card has a minimum of 500W and the old card requires 450W so would my 600W psu still have enough overhead or would more be recommended?
Seu novo cartão usa cerca de 100w. Um PSU de 300W seria recomendado, qualquer valor maior seria desperdício e ineficiente.