since your building your rig
look into a CrossFireX with 2 5870s
that will keep you happy for a while
5870 is an epic ripoff if you know what you are doing. Here is what I recommend:
1.) get a reference model 5850 ( does not have to be reference but reference is guarunteed to work). Afaik, the manufacturers where you wil gt a reference model for sure are visiontek and his.
2.) use gpu-z to back your bios up. Next to bios info, there is a button that allows this. This step is very important if something should go wrong.
3.) acquire a bios from a reference model asus 5870. Techpowerup has an extensive database of vBIOSes, so you will most likely find it there.
4.) use atiflash to flash the 5870 bios onto your card, then reboot.
If all goes well, your card should now he detected as 5800 series, still with 1440 sp's but it will now run at the same clocks as a 5870. You can confirm this in gpu-z while a 3d applicaion is open. At the desktop, your clocks will be incredibly low because ati throttles it in order to save power and reduce heat. Performance should be increased by a pretty good bit, and your card will perform within 5-10% of a true HD 5870.
If something goes wrong and you end up with a black screen and/or the 'checkng nvram screen' then what you need to do is either put a different card in the top pci express slot, no crossfire bridge, with yours in the bottom, or remove the card, plug a monitor into the IGP (onboard graphics) and set the bios to initial the igp first. If the board is really old, you may not be able to do this, and will have to resort to the 2 card method.
Now, using the bios file you backed up, use atiflash to reflash the card back to stock. It should now be working again, and I suggest trying a different 5870 bios, since the one you got may have been corrupted.
I have not done this personally, but I have run into a few people who did it, and they all got good results.
