I wanted to chime in on the PCIe dealio. Unless you're planning on playing games, the X1900 would run just fine on a 2x lane. The amount of data transfer required to render the finder is not that high. On a 4x lane you'll get a performance hit in games in theory but the high resolution of the monitor would be a bigger issue.
Also as far as Crossfire/SLI goes, the only deciding factor on if/when those technologies could come to the Mac is wholly dependent on ATI, NVidia, and Apple. There's no technical reason why it can't work, nor is there anything special about the SLI chipsets that makes them a requirement for SLI. In fact, ULI (part of NVidia now) has a patch so that you can run SLI on their boards. There are also hacked drivers out there to allow SLI to run on other non NVidia chipsets and "unsupported" Nvidia chipsets.
All that needs to happen is the creation of proper SLI/Crossfire drivers for the Mac. My guess is thats a low priority for ATI and Apple at this point.