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.
right, yes i bought a new powers supply as well its 500 w sould be fine with it! my mother board is: http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01...
and i got cpu ''CPU Type TripleCore AMD Phenom, 2900 MHz (14.5 x 200)''
and current onboard graphic adapter nVIDIA nForce 6100-430, AMD K10
Any PCIe card will work in that, the trick is getting one that is supported by the PSU. That is a relatively old processor. It still has game, but I would not pair it with anything more than a $150 GPU.