That is a perfectly normal temperature for stock cooling on the HD3870.... if you would like to lower it I would suggest getting a Thermaltake DuOrb
Newegg.com - Thermaltake CL-G0102 VGA Cooler - Retail
I used 2 of these when I had HD3870s in Crossfire, they fit just fine over the stock RAM sinks on the 3870, which is actually quite a nice piece of copper so just leave it on there...
Also at 23C ambient, temps idled around 32/35C (seated with MX2) and loads never broke 60C even on crisis.... Now thats just one fix, as for the fan fix bios....
Refer to this guide on how to make a boot driver and properly flash your device....
techPowerUp! :: Guide to Video BIOS flashing
I personally use the USB Drive method found here....
techPowerUp! :: Guide to Video BIOS flashing
And use ATIFlash 3.49 found here to flash the bios you want...
techPowerUp! :: Download ATIFlash 3.49
Once you do the Boot Disk (however you choose to do it) you will need to extract the ATIFlash files to the storage media your using to flash from.... and then reboot your computer and either use a BBS Option (BIOS Boot Specification), or just go into your BIOS and change the boot order if your motherboard does not support this.
After your device is booted, it should load to a DOS prompt, where you will type
Code:
atiflash.exe -f -p "device ID" "BIOS.bin"
Where Device ID will be 0/1 (without quotes) depending on how many cards are present. More than likely you will be using 0 for a one card setup. And as for the BIOS.bin, this will be whatever the name of your BIOS you wish to flash is, without quotes of course.
For an explanation of different commands you can just type atiflash.exe at the DOS prompt and it will list them. -f -p is a command to force BIOS flashing regardless of various checks.
As for the "Fan Fix" bios you can find it here....
Rage3D Discussion Area - View Single Post - HD3870 860 core fix + fan speed fix
It also removes the 860mhz PLL Divider problems from the original 3870 BIOS, as well as the thread includes the original stock 3870 BIOS. I would suggest downloading both BIOS's so you have them on hand if something goes amiss in the BIOS flashing.
also if you have never done this before, please save yourself the hassle and have a secondary VGA card at hand if something goes terribly wrong, so you can at least get back into your DOS prompt to reflash back to an old BIOS and recover your 3870, or whatever you are flashing.