I pulled apart my Socket 478 HSF, the fan is identical in size, except thickness, as the Socket AM2 X2 stock heat sink's fan, so I fitted it to the AM2 X2 stock heat sink using longer screws and it's currently running at just under 1800RPM, which is what the 3500+ HSF ran at.
I fitted the fan that came stock with the AM2 X2 HSF to the socket 478 heat sink and it runs at (about) 1300RPM on that board

, it wouldn't go passed 890RPM on the AM2 board.
In any event, I've solved this issue with a little elbow grease.. this computer (AM2 X2 3800+) is now running at 29c, goes from 25-31c idle, cuts off at (so far) 39c under heavy load.
I don't have a huge case, it's the standard Acer T180 mid-case, 2x 5.25", 2x3.5", 4xHDD bays.
I also unhooked the intake fan, it sounded like a 747 trying to land on my desk! .. I also removed the exhaust fan, so now I've only got the cpu fan going, and it seems like it's enough.. it keeps my dual core nice and cool
Anyways, thanks for the help and I'll end up getting a better cooling solution later on, when I upgrade the cpu again.. I just wish AMD didn't stick a 4 pin PWM fan on the HSF combo.
I should also note that I don't have an aftermarket video card or a whole lot that would heat the case up, and my cpu is a 65w version of the 3800+.. so results probably aren't typical.