Ahhh...about 2 hrs too late...just spent the morning searching for ROM for a HD 4870.
I had a look but could not find my card via the part number on your link
Part number 288-20e85-130sa
Sapphire ATI Radeon HD 4870 1Gb GDDR5 PCI-E Dual DVI/TVO
works with firmware posted on this post
https://forums.macrumors.com/threads/661681/
Fan does not seem to come on..but did not test for more than two or three Benchmarks/ stress tests.
3 D performance much improved with this card compared to ATI2600..but no improvement with Photoshop speed..so I guess something else is causing my Photoshop speed bottleneck..Scratch disk. / SATA bus bottle neck??
But do we need new ROMS if we use Zeus? Zeus instructions say Zeus makes new firmware from the old firmware
From the instructions on the Zeus page.
On the Zeus download pages it says that you take the old ROM off the card while in Windows, send the rom firmware back to the mac and then re load it in Zeus. It implies there is no need for a new firmware ROM, since Zeus makes the new one from the old one.
I tried but it did not work,
Made new power cables, connected to CD drive spare molex
Managed to find a ROM image here for the Sapphire card I had. Successful flash with Zeus 2.8 from within Mac 10.6.8.
All fully recognised, both DVI ports working on the first flash, fan never seems to work...tried varios fan settings in ATI custom rom settings tool.
But no graphics performance increase in Photoshop..
3D benchmark showed good increase 380 fps..compared to 120fps with the ATI HD 2600 XT. but the 2 D performance when using Photoshop actually seemed slower.
So have removed the HD4870 and gone back to the 2600. no pint in using all that extra power for no benefit.