1) pavillion range from old pentium 2s all the way to brand new computers, so we need to know which one you have in order to help
2) I can almost guarantee that you will need a new power supply. The 280 is a bit of a monster and you may have issues getting things to fit proper depending on your case (the cards are really long, so it physically may not fit). Go to nVidia's website to see what the suggested power rating is for a system with your card and get an 80+Bronze or better PSU that will work with it.