We were in the same boat man. I wanted to use my the XP-90, but since Monarch didn't offer it I just got the OEM CPU and the XP-90 somewhere else. When the CPU arrived with a heat sink actually attached I was kinda shocked since I didn't order one (don't worry they didn't charge me extra) and I really didn't know how to take it off. But it's actually relatively easy.
You can use your hands to unlatch the thing that holds it down, then use a flathead screwdriver to undo the two clips on each side (be careful, don't slip and break anything). Then gently TWIST (don't pull directly, when I did the CPU came out with it and scared the f outa me) the heat sink off the CPU and wham, the heat sink is off.
Now you have a mess of thermal paste everywhere, use a paper towel or something to clean the bulk of it up, then use any kind of rubbing alcohol (doesn't have to be isopropal, I just found some around the house to use). Actually you can even use water for this, but alcohol gets it cleaner.
The stupid part is the backplate. Thermalright gives you a backplate to use instead of the conventional AMD one, use it. Unscrew the screws, remove the backplate from the motherboard, screw the new one on. Pretty self explanatory, but for some reason the screws for the AMD backplate are too long for the Thermalright one, so I had to use some nuts to compensate.
Dry it someway, (paper towel, air dry, whatever), clean the bottom of the XP-90 off the same way, apply some new thermal paste (I have no clue what a thermal pad is, most people use thermal paste because I bet it works better but I would imagine a pad being easier to remove), spread it around with a flat edge (like an old credit card), make sure the whole thing is covered, smack the big XP-90 on, clip it down, get a fan on it using the right sized clips (the XP-90 comes with 92mm ones and 80mm ones) and bam. Have fun!
Anyway I would get regular thermal paste, preferably Arctic Silver 5, instead of a thermal "pad." I don't trust a thermal pad (although I'm sure it works).
Wow that must've been the most in depth guide to "replacing the heat sink" ever. Personally though I think the heat sink is the hardest part to building a comp.