Can someone please comment on the following statement:
If R430 is a revision of R420, then the R430 should be able to run a R420 BIOS. Similarly, an R480 should be able to run a R420 BIOS. (Assuming core and memory timings are compatible).
Since the X850XT has higher core/mem clocks than an X3, then it should be able to run the X3 BIOS that has lower core/mem clocks. However, since the X800XL has lower core/mem clocks than an X3, you will not be able to run the X3 BIOS, unless the X800XL overclocks well.
With a BIOS editor like RaBit, you could take the X3 BIOS and decrease the core/mem to XL speeds. Note (1) that the X3 has 12 pipes, whereas the XL and XT have 16. I'm not sure what will happen... if the card will run with 16 pipes or if an additional BIOS edit is needed to make it use 16 pipes and therefore be even faster than an original X3! Note (2) Mac versions of X800 have duallink DVI which allows you to run hi-res TFT screens like Apple Cinema 30" Display. PC versions of X800 do not have duallink.
Please report your results!
Follow this thread for an X3 BIOS,
http://forums.techpowerup.com/showthread.php?t=16440