chances are he re-seated the ram, and the reinstall of the OS probably fixed some file that had gotten corrupted or was acting screwy (or maybe you had a 1/2 installed patch or something along the way...)
In general, those should be 2 steps you should talk yourself. I honestly have found apple to be VERY good to deal with... MUCH better than any of the other machines I've used (even the dells we have at work with gold care support...)
I've had 2 HD crashes on my macbook, and in both cases, looking back on it, it was because I had been using it while in the car (I wasn't driving) and/or not waiting for it to go fully to sleep before moving it around... In both cases, they said "Ok, bring it into the store, there's an opening tomorrow morning." I did so, 30 minutes later I had a new HD installed in the machine and my TM backup was being restored... No other questions asked.
When I bought my new 2009,1 Mac Pro, it wasn't even a consideration whether or not to get the applecare... it was an automatic "You BET I will!"
Rather pay the $$ and know I have the support than wish I had bought it later when or if something goes bad...