My computer (HP p6510y) came with an integrated card (Radeon RS880). I was told I may need to delete the drivers for the old card and move a pin over 1 space on the mobo before I install the new card (Radeon HD 6750). I am getting ready to install my new power supply, but that should be rather straight forward).
I do not see any mention of a pin or jumper to move, the onboard video card should self disable once the system detects the new card.
As for changing out a power supply, it's not that hard. Just remember you need 2 power cable sets to connect to the board. 24 pin and a 4 pin(many new power supplies have this as an 8 pin connector that splits into the sets of 4 and only one side will fit).