reference cards work best
the closer it is to the Apple one, the better
so early 512 Meg cards with the reference fan are best bet
EARLY XFX cards with part numbers like "ZHDC" or "ZDDC" are best in the 1 Gig world. And again, if you look at pix of them, they are very similar to early Apple cards, but different coloring.
The reason the "ZWxx" cards were originally recommended was that they originally worked. MOST 4870s flashed beautifully but then 10.6.3 happened. Ever since then, MANY 4870s aren't good flash candidates. The further they strayed from reference design, the more trouble they became. If the fan has only 2 wires, RUN away.
I have flashed well over 100 of these 4870s in last couple years, I know what I am typing about. With any flashed card, it is always best to stick with one close in design to original card the ROM came from. This makes sense when you think about it for a minute.
It is unfortunate that the 4870 flash guides were never updated since 10.6.3 brought mayhem, but that is how it is. Netkas and Keita wrote patches that contained earlier driver versions to limp these cards into future OS revisions, but I don't know if anyone has bothered for awhile.
But as pointed out, 6870 is a better choice for many things, especially if you have 3,1 or newer.
This board was the place for 4870 info. The best flashing info has since moved to Netkas' own board. He does the bulk of the ATI figuring out and has almost as low a tolerance for foolishness as I do.
When flashing a card, it is best to REALLY read and compare notes and THINK before buying up a card and hoping that someone with experience will come along and figure it all out for you.
Cindori's Zeus even works best with early 4870s and in many cases can do everything in a single reboot. But only for 1,1 and 2,1 Mac Pros would I seriously recommend a 4870. The 5xxx cards flash pretty well but boot screen is difficult to see. The 6870 cards are flash able for EFI64 only but will work in 1,1 and 2,1 with some limitations.
It is likely that after Mac Pro 2011 (or 2012) comes out with newer ATI cards the there will be a flash for 1,1 and 2,1 machines. The 6950 will likely even became mainstream then.
Read & think and you will end up with a card you like.
Or, pay Apple $450 and get a 5870 that has no issues at all.