The contradicting information may result from the fact that ootb support is not complete, so you would still need to flash for things like e.g. a boot screen directly off that card (see below for more details).
Not sure about the PC-ootb-5770, but the 6870 offers more power (~ 10% below the 5870, but with a significantly lower energy consumption) for roughly the same price - and it does work out of the box for sure (even in a 2006 MP 1,1), provided you have either Snow Leopard 10.6.8 (or higher) or Lion installed.
Limitations:
Ootb the 6870...
- ... won't show a boot screen.
- ... will crash when trying to play Steam-powered games.
- ... won't play back DVD's using OSX' DVD player.
Mitigations (without flashing):
To get a boot screen you need to have a second graphic card installed in the system. Any simple "native" OSX card will do, e.g. the factory-installed 7300GT. Alternatively you can set the boot device in OSX' settings, so you don't necessarily need a boot screen. For the login window the card will show a picture eventually.
For Steam-based games and DVD player to work you could install ATY_Init.kext by Netkas. Drawback: the second monitor port ("upper") won't work anymore, so running multiple monitors off that card may become a problem. Additionally it's no official solution, so the next major system patch .x may break it at least temporarily.
Alternative mitigation for Steam/DVD playback:
Use another player (DVD playback).
Wait for an updated Steam software that may(!) cure the problem.
