If you have "dual channel" memory then you need to add 2 x 512; otherwise 1x1gb will be fine. Take a look at your motherboard manual for best advice. IF you don't have it, click here for SIW which will ID it and google will find you a manual.
You should not need to reactivate after adding RAM. I've not had to at least; as for adding a new hard drive, I'm not so sure.