new HD !!

The first thing to consider would be the price difference. I'm not sure how much hard drives cost in you area but in my area, a 200GB and 400GB will only have a difference of about 20-30US dollars. Considering the small difference in price, I would suggest you buy the one with the bigger capacity. You will be spending more but you will get twice the storage capacity. With a bigger storage, you would be able to store a LOT more things.