Unfortunately requirement 1 and requirements 2&3 are pretty much contradictory. The current Mac Pro only has 2 PCI-e x16 slots, and there are only a couple of compatible GT200 based cards with EFI support (unless you try flashing a PC card with an EFI bios or do some other boot loader tricks). Further to that the current Mac Pro only has about 300W available for PCI-e cards, which just about rules out a pair of GT200s. More power requires hacking or upgrading power supplies. There is rumoured to be a Mac Pro refresh due early next year with 6 core Nehalem processors, so the motherboard and power supply situation might change.
Also, there is presently no 64 bit version of CUDA for Mac OS X, so you are limited to running the 32 bit kernel with 4Gb address space, which can be very restrictive depending on your application. That might also get fixed in CUDA 3.0, which should be released soon.
If your need is hardware today, and are developing your own app rather than using someone else’s and 2 is the most important requirement, then seriously consider 64 bit Linux (as unpalatable as it might sound), where you can easily drive 3 or 4 GPUs on stock PC hardware with 8 or 16Gb of ram at a fraction of the price.