So wierd because I have the same CPU and I haven't put a AC cooler on this yet and my temp runs at about 34*C-42*C (OEM heatsink and fan) but I also have it in a Zalman GS1000 case, lots of airspace and a killer layout for cooling.
I'm sure you already looked into dust being the issue but if you haven't you should look into that, sometimes dust will cake onto the fins or fan and insulate it from doing it's job properly.
I would have put a better cooler on this already but the OEM is doing a great job for me, plus I plan on getting an i7 soon as I can so I don't want to put money into something I'll be getting rid of anyways.
-D