I think things are getting a little confusing.
The 4890 cannot be flashed for mac, yet (this depends on a number of things, particularily on whether apple releases an oem 4890 of their own, AND how big the rom size is as well).
The ONLY way people are using 4890s in the MP is by using "injectors" which also requires ANOTHER graphics card to be installed (for booting the EFI). You also need to install of installer files in OSX.
The other graphics card MUST be a apple oem card with EFI. I'm guessing you are running a 2009 MP, so ideally you would want an GT 120 or ATI 2600, as those do not require aux power connectors. Before when the first couple of batches of 4890s came out, it came out with 2 x 6pin PCIe connectors. However recently it seems,(i.e. xfx 4890) certain revisions have changed the power to 1x 6pin and 1x 8pin. This poses a problem of using the MP aux power connectors. Make sure to double check how the brand 4890 you buy is powered.
There is another way to power by using the lower optical bay power; using 6pin pcie to molex. However ever again, watch out for the 8pin versions, as you will need to buy an adapter accordingly. Also just to note, while people have had success with powering from the lower optical bay, it is more "safer" to power from the above mentioned aux power ports.
Here is the thread about running 4890 in MPs:
http://forums.macrumors.com/showthread.php?t=713428
Again, you will need another graphics card, so plan for that when you buy the 4890. Also note, you will not get dual dvi functionality, however since you have another card in there already its not really an issue I guess.