1) you probably wont notice much difference, theres not alot of headroom on the 5200.....its just to weak. But any improvement would be better than what you had so give it a shot and see what happens, you may just have the mad OCable 5200.
2) yes that would help, anything active (with a fan) will help alot. You could also try attaching a fan to your cards heatsink, get creative. Think zipties..........
3) to attach the AMD sink and fan you would first have to remove the other heatsink. If its thermal epoxied on this could be a chore. I have heard of people putting the card in a plastic bag in the freezer, this loosens the epoxy and it should pop off easily. Then use some thermal epoxy and stick the AMD sink on it. If this is your only graphics card and you cant afford to replace it I suggest you not do this. You could pull the whole
gpu off the card when attempting to remove the old heatsink, it happens.