I think that when AMD say their CPU will work to 90 degrees - they mean it won't instantly burn up below that. Above 50-60 degrees expect (although difficult to proove) shorter cpu life.
Also, if your case temp is up in the high 30s - what about everything else in your system. HDDs will fail more often, new video cards are hot - sometimes really hot, CD drives will have more errors. Generally good computing temperatures are 18-25 degrees C. That's what you'll find most datacenters at. They are at that temp for a reason - it reduces wear and tear on hardware.
Retail HSF on a 133 will definitely be your problem. Get a decent fan - there are enough reviews around at the moment - and for the most case, use artic silver as your compund, not the little pad on the heatsink.
Not sure about your case, but my system has 4 x 80mm fans (2 in, 2 out) and can just about keep things in the high 20s at full load. I think you'll need a good system with that 133 in there.
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