use cotton swab over the cpu and gpu and heatsink till you get all the muck off ( may be old paste on cpu you get used) dry it off and heatsink then apply mx4 2 dots on cpu and gpu put cooler back on usually tighten in order of 1 and 3 and 2 and 4 if screws a lil each time this keeps the heatsink on properly
would work same temp and power usage
thermal pad may be under heatsink remove it gently and stick it on the new cpu if non is there dont worry