You will need around a 400 watt power supply for this thing, which is pretty dumb, since you already have to spend 500 on the card and another 100+ for a psu. Good call on nvidias part.
I think even HP PSUs are standard sizes. However if you don't have the good stuff in there already which you may not have if it is an HP, then upgrading to an 8600 may not make sense if your unit can't take full advantage of it.