how well did you get the stock intel cooler to seat? the fan assembly attachment is wimpy and its easy for the prongs to pop out. i would be afraid to overclock on a stock intel fan assembly unless i was absolutely sure it was seated properly and was not going to pop off at a later date.
with turbo boost enabled i do not think you would need to overclock the cpu unless she is going to use her machine for something more than email, surfing, and watching movies.
as to your question about the difference between 650 and 680... i believe you are right that the 680 is the 650 with a faster speed not sure if its done through different multipliers or not.
I have been running my 650 at 4.1 ghz on air since i have bought it in may of this year. All i really did was bump the bclk.
If you are sure the machine temperatures are stable then i would not be afraid to overclock it to 3.6