Yea to be honest I am very new to Macs.. So for me it was a rather painful process to find out what the deal was.. My main problem was getting a boot cd and the flash files on the same CD and make it usable.. I eventually just formatted one of my drives as a FAT partition and that worked. Once I had the method it was no big deal.
I just bought my Mac a couple days ago.
What you need:
-DOS boot CD or MS-DOS or USB dos boot something that will recognize a internally formated FAT partition with the flash program and the rom..
Boot CD I used
http://gog33k.com/MS-DOSBootCD.nrg (it is a nero image)
-nvflash.exe (flash program)
-mp8800gt.rom (mac pro rom)
-8800GT with 1024 kb of flashable rom memory.. (which you can check with nvflash)
-Power cable for motherboard if you don't have it from here
http://shop.ati.com/product.asp?sku=3280778
You can also use the split 4 pin cables to 6 pin if you have it and use your extra CD rom power cable to power that.. but you can't close the cover that way.
Optional -
If you have another ATI video card or some other card you can put in as well I think that would be helpful if your flash does not take to flash it back.. Though I had no problems. If your other card is another nvidia you will need to specify which card you are working with the nvflash commands. like "nvflash --index=0 -4 -5 -6 mc8800gt.rom"
the --index=0 would be your first device..
Warning: Someone on some of these forums suggested booting with one video card and taking it out while the machine is running and putting in another card.. I cannot stress how incredibly dumb of a idea that is for what should be obvious reason of damage to hardware and electrocution.
Download the files for the flash and MacPro rom
http://www.palmirapdc.ca/8800GTflashtoMac.zip
Instructions..
1. format a hard drive to fat with disk utility
2. copy the flash and rom files to the hard drive. Make sure you uncompress them. 3. boot from dos CD
4. c: (or change to wherever the files are)
5. nvflash --check (around the last line it will show you how much rom memory you have to work with you need 128Kx8S, 64 will not work..
EXAMPLE : NOT GOOD = EEPROM ID (9D, 7B) : PMC Pm25LV512 2.7-3.6V 64Kx8S, page <-- the key part that was important was the 64K you want it to say 128K
if(you have 128K)
6. nvflash --save BackupofOld.rom (backs up your old rom)
7. nvflash.exe -4 -5 -6 mp8800gt.rom (flashes new rom you will be prompted just select y for yes over and over..
That should be it..
If it doesn't work flash back to your backup
nvflash.exe -4 -5 -6 BackupofOld.rom
List of working cards
millisecond: PNY VCG88512GXPB green pcb 128k
aLoC: Inno3d 64k
aLoC: Inno3d reference model (no overclock) 128k
smgfreak: PNY VCG88512GXPB 64k
SuperGrobi: XFX 8800GT alphadog XXX PV-T88P-YHD4 64k
AGDenton: Gainward 8800GT 1G red pcb 64k
smgfreak: Sparkle 512MB 600MHz 64k
smgfreak: PNY VCG88512GXPB 64k
mrcandy: EVGA 512-P3-N801-AR 128k
giantsfan1975: E-VGA 8800 GT 128k
smgfreak: Sparkle 8800GT FEB 2008 REV 1.2 64k
abisai: E-VGA e-GeForce 8800 GT Superclocked 128k
smgfreak: ZOTAC with blue pcb 64k
rhildinger: BFG BFGR88512GTOCE 128k
antsh: XFX PVT88PYDD4 alphadog 128k
vortigern: Inno3d i-8800GT-H5GTCDS green pcb 64k
smgfreak: LEADTEK PX8800GT 512MB ZL (A JAN 2008) 128k
AGDenton: BFG BFGR88512GTOCE 128k
SuperGrobi: XFX alphadog PV-T88P-YDDA black pcb 128k
hadleydb: BFG BFGR88512GTOCE 64k
hadleydb: MSI NX8800GT-T2D512E-OC Nov. green pcb 128k
bartdesign: eVGA 8800 GT KO 512-P3-E802-AE 128k
inCider: eVGA 512-P3-N801-FR (image problems) 128k
inCider: XFX PVT88PYDE4 (image problems) 128k
thagomizer: PNY VCG88512GXPB 64k
slackpacker: PNY VCG88512GXPB 64k
HansG: MSI NX8800GT-T2D512E-OC 128k
Spybreak: XFX alphadog PV-T88P-YDE4 black 01/08 128k
Sources-
http://www.reduser.net/forum/showthread.php?t=9689
Probably more detailed..
http://forums.macrumors.com/showthread.php?t=434092&page=10