PCI-X is also older (older than MAC's inclusion), and slower. I have PCI-X on my ASUS A7M266D mobo. I THOUGHT it was the wave of the future but it's mainly used for large RAIDed HDs and Gigabit ethernet right now.
PCI-EX is a different and there are even different flavours of it. The future PXI-EX connectors will be smaller, but for a while they will include both PCI and PCI-EX on the same slot for easy backwards compatibility. Some computers will ship with all 3. Some mobos were previewed with 15 PCI-EX connectors on one mobo.
The main difference is that PCI-X was used in conjunctions with all the other connectors (my mobo has 2 different PCI-X slots and 3 PCI slots, and 1 AGPpro slot, PCI-EX should REPLACE all the other connectors.
Apple's adoption of PCI-X seems VERY shortsighted IMO. However PCI-EX is kinda more an Intel thing so I giess they didn't want to support the 'enemy', even though that's harder to decide who's who with IBM and MS so integral to APPLE's strategies.
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