demitri, the power cable that is/was plugged into your stock X1900 should work just fine. In fact, that's exactly what I did: roll one myself, and take the other from my old, dying X1900.
So, I can finally report success, too, with an XFX HD4870 1 GB card (the standard version, not the XXX one). They're still using the reference design, or a board very close to it. However, this card does not work with Apple's ROM nonetheless, and neither does it work with the ROM yielded by pipomolo42's method without modification. Been there, done that, thought I bricked it. Apparently, XFX's ROM is slighly larger that the others, which means pipomolo42's script overwrites some crucial part. After all else failed, I decided to reread the instructions. On page 20 I stumbled upon the ROM kindly provided by rhildinger, which somehow escaped my memory the first time through. So I flashed that, and, lo and behold, the card works.
Not only that, but I have dual DVI support, on 1920x1200 and 1600x1200. I can't test dual link or VGA, though. Unfortunately, whenever I connect my older monitor, whether as sole, primary or secondary screen, I get the mouse jump issue. I guess we can add the Fujitsu-Siemens P20-1 to the black list. My brand new Iiyama monitor is fine. Too bad I can't test whether the mouse jump issue is absent with two 'good' monitors. Any news on this front?
So, special thanks go to rhildinger for his ROM. Of course, thanks to all the others who made this possible, too, among them pipomolo42 for the hybrid ROM method, netkas for the drivers, v0n for instructions on the power cables, and The Rominator for getting te ball rolling, so to speak. By the way, rhildinger, if you're still reading this thread: am I correct in assuming that the card runs at Apple's lower clock speeds in OS X, not at XFX's overclocked ones, with your ROM?