Here are some other images, more in Apple style:
slim tower (with 2 PCIe slots, I suppose):
mini cube (PCIe slots not shown!):
Keep in mind that full-sized PCI cards are 12.28" long by 4.xx" tall. Half-sized PCI cards are 6.6" long (could fit in a 7"^3 or 8"^3 enclosure). Still, something smaller than 10"x10" would need a custom-size motherboard/power supply, while there are lots of microATX motherboards/power supplies, and I am sure that Intel would be please to rework one of their motherboards to meet Apple (our) requierements...
I wouldn't mind the mythical xMac to be bigger than smaller, I can certainly imagine it being neither a cube or slim tower as shown, but half-a-cube of 13"x13"x6.5" that could accomodate:
- a microATX motherboard G31/G33/G35/Q35/P35 chipset, C2D/C2Q, 4 RAM slots (up to 8GB)
- One 16x PCIe slot and Three 1x PCIe slots (or One 4x slot for the slim tower design)
- One desktop optical drive
- Two desktop 3.5" hard disk drives (if not four, depending on the interior design: 2 on top below the ODD, and two at the bottom, if you start from the Mac Pro interior design)
- the usual complete range of ports (usb2/FW400/FW800/Gb Ethernet/audio...)
If I was in charge of the specs, I would make it with integrated graphics standard because not everybody needs dedicated graphics, and those who need it could still add a dedicated card), I'd use one of the chipsets mentionned above, so that it would allow for dual and quad chips and compatibility with the upcoming penryn desktop chips (at least for the G33/G35 chipset).
I would offer a barebone configuation at the free $999 price spot, two better/best configurations at $1299 (quad-core) and $1999 (EE quad-core), and offer all the BTO options of the Mac Pro and more...
If there were more PCIe Macs, manufacturers of PCIe cards would create more compatible drivers for the Mac and make more PCIe products available, including graphics cards. Ditto for Expresscard products by the way. Too bad the new iMac doesn't offer one Expresscard slot.