My Arctic Cooling Accelero S1 and Turbo fan kit arrived today. Took a bit of time to install what with taking off the old HSF and cleaning up the GPU and VRAM modules. Any way just testing it under Windows now (the only way I can see the temp of the card as I don't have a heat probe) and it is really cool.
Idling at 33oC and fully loaded only 45-46oC
This is about 20-30oC cooling than with the stock cooler. (nice)

Will test it in OS X later when I get back from work and will post the temp I see when I log back onto Windows. Before, after using OS X for a few hours, when I logged back into Windows the GPU was 70-80oC (ouch). I am hoping it will be a lot cooler now.
My Accelero hasn't arrived yet, but I think we're doing OK with the latest Natit and White Menu fixes. I picked up a Kama Thermo (
http://www.newegg.com/Product/Product.aspx?Item=N82E16811998012) with another order, and jammed it deep inside the cooler's innards.
In XP (at the probe), it idles around 95oF, and in 10.5.2 it's about 100-110oF. In XP, the driver says 107oF, so I expect I'm at about 120oF idle in Mac.
Different, but I assume since Mac has eye candy always "on" and XP is running on "best performance", it isn't too bad. On Mac, in WoW, I'm getting 160-165oF (fan speed NEVER increases) at the probe, so your 70-80oC is about right-on with the old "always-on" driver.
It's actually cool to the touch at idle now, which it wasn't before. I don't feel like I *NEED* the Accelero, but it's ordered, so what the heck. The stock cooler is a bit of crap, so why not. If I had a 3870 from the start, I'd probably not got the Accelero since I think the stock is copper with a heatpipe and exhausts out the back.
I tried some undervolting with stock clocks with AtiBiosEditor, but never achieved stability (3-D would crash). I fiddled with the power states in the Lamna info.plist (changing the low->high percentages to stay at low more often, etc..), but video playback suffered (Camera AVI's would play at like 5-10 FPS in Quicktime), so I'm back to plain stock 3850 BIOS.
What did you use to clean the old card?
-Dan