If you buy retail parts, you still get the manufacturers warranty. The reason one would buy Monarch's warranty for a retail CPU/MB, is that you can RMA thru Monarch during the warranty period vice the manufacturer.
If you go OEM, I would suggest the warranty as insurance. You might never need it, but you might. I had my retail 3000+ die on me, so it does happen.
About the thermal grease, don't buy it!! They will put it on for you, but they DO NOT send the rest. You are paying $14 for a one tme use. IMO that is complete BS, especially considering that they do not let you know that up front.
So if you ever need to remove/reinstall the HSF, you'll have to buy some more paste/grease anyways. Just buy some AS5 or Ceramique from jab-tech or SVC and put it on yourself when you receive the combo.