It's probably because the Mac Pro 2006 edition has PCI-E 1.0 and the new cards need PCI-E 2.0 specs (
http://en.wikipedia.org/wiki/PCI_Express#PCI_Express_2.0). It's just technology doing what it does best... making it self obsolete every 6 months.
That's a lot of bollocks, unless Apple seriously screwed the pooch somewhere.
From ATI's page:
ATI Radeon™ HD 4870 System Requirements
* PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
It's typical Apple BS. They're too lazy to make sure the card is EFI32 compatible, so they say "screw you" to the guys who buy their stuff. I paid for the 8-core option before it became standard, and I'm genuinely sick of being treated like a second class citizen by Apple for helping keep their sorry asses swimming in cash.
You'd think after a quarter of a century I'd have learned, but apparently I'm just too optimistic.
No longer. I'll relegate my Mac Pro 8x3.0 to being a render box, and I'm sure as hell going to stop recommending Apple products to people. The Mac Pro was the last machine I considered worth the money, and it's clear now that Apple doesn't care about the buyers as much as they do for the ****s who buy their consumer-line junk every 8 months when it gets a minor tweak. I'm surprised those douchebags haven't made the 30" into a glossy panel yet, since they're doing everything else they can to annoy people who actually use these machines to make a living.