If your fan control on your 3870 does not work, here is how to flash it:

Some files: here

-DON'T USE ATIWINFLASH.exe. Your system will hang. Your card will not work, and you will have to do it with another or integrated video card. Use atiflash.exe

Backup your bios
-use gpu-z to backup your video card bios. Click on bios chip icon near bios version in gpu-z. (save to file, ending in .bin)

Modify your bios
-download program RBE_128. (Radeon bios editor.)
-load bios. Find your .bin file.
-go to fan settings
-click on "set all fan setting to recommended values".
-file/save bios. Save as file, ending in .rom.
-exit program

Flashing your video card
-find and download atiflash_386.zip.
-download boottoolkit from vuks.vdsworld.com.

-Create iso with atiflash.exe and your modified bios file-
-unzip atiflash.zip
-unzip boottoolkit to d:\temp
-copy atiflash.exe to D:\temp\flash\boottoolkit\files (assuming you unzipped boottolkit file to d:\temp)
-rename your saved bios from "2D - 300, 900, 1.134 3D - 825, 900, 1.214.rom" like name to "myfixed.rom". Make sure it has the same ending as the original name, which is .rom.
-copy myfixed.rom to D:\temp\flash\boottoolkit\files.
-delete bootcd.iso file, if it exists, in D:\temp\flash\boottoolkit\.
-Click on make_boot_iso.bat, to create new bootcd.iso.
 (alternative method is to use Ultraiso to modify bootcd.iso from boottollkit package. Atiflash.exe and myfixed.rom should be at the root of the iso file.)

-Flash from dos-
-Burn iso to the cd, or cdrw.
-If you have UPS, connect your PC to it.
-Setup PC, in bios, to boot from cd.
-Boot pc.
-Select "Boot with CD support"
-after booting, type C:, or D: to get to atiflash.exe
-use "dir" to display direcotry content, "cd newdirectory", to change directory
-to flash card type this:
 "atiflash -i" will give you info on ati cards present. First will have id 0, second 1.
  If you have one ati card, it's id is 0
 "atiflash -p 0 myfixed.rom" will flash your adapter with myfixed rom.
 "atiflash -p 0 myfixed.rom -f" will force flash your adapter with myfixed rom. I used this after first unsuccessful flash. Second one was successfull. :)

It is best if you can make flash from your own card. If not try to find one from the same version/manufacturer video card,  and/or the one with the same bios size.

Good luck.

-You can change video card fan speed and memory speed with Riva tuner. Ati tray tools did not work for me.
-In Riva tuner go to the main tab. Click on the triangle near Customize. Select first icon.
-Go to fan tab.
-Enable low level fan control. set speed to auto and enable "Apply fan settings at Windows startup."
-You can watch fan speed temp with gpu-z.
-You can monitor/test your gpu with with Furmark. Also Gpu-z comes with the same package.

Instal monitor drivers if you get out of sync problem (unrelated to bios flashing.)

Doing it with another video card
Here is how:

I used pci card (really old S3 graphic card. It goes in the same pci slot that separate lan card, modem, or sound card goes,

Go to bios of your pc. I used Asus motherboard. In the pci? settings there is an option of which video card you want to initalize first, which pair to use.
I used pci card, and my Ati card was PCIEx So my bios was set PCI/PCiex. PCI is old S3 card, PCiex is new 3870. I could not get video when it was set to integrated/PCIEX. I could see things on monitor, but, atiflash could not find ati adapter.

If you have motherboard with Integrad video card, then you shoud use Integrated/PCIEx order, I suppose.