are you sure your shifting when the clutch is fully depressed? if it was your clutch you would probably experience the grinding sound on more than just one shift. what about 1-2? if it's harder to get into gear than it should be it's most likely a clutch thats not fully disengaging, but if it's just one shift it's possible that you have a habit of shifting before you get the clutch off the flywheel.
how many miles on that car? It's hard to believe that the previous owners could burn out two clutches that quick.