While the '08 may be easier (and admittedly more pleasant) to work with, Apple has shown in the past their almost nonexistent interest to support even slightly aged hardware. Just look at what we had to go through to get them to crank out a legacy 8800 GT.
It contains EFI64 though, and wouldn't be subject to the same issues. Assuming Apple doesn't find a way to futz with the firmware (i.e. locate and use another standard to proprietarize), EFI64 will be around awhile. So will K64.
So hardware, such as graphics cards will be able to work properly. Perhaps changes to the power specifications could create difficulties (i.e. all cards go to 8 pin PCIe power connectors, and the boards can't handle the current). But there would still be options to get around it, such as a separate PSU used for graphics cards. Other items set for Mac, particularly EFI boot capable, would work (BIOS too, if used via drivers in OS X or to boot in Windows). OS X will continue to be updatable for a few years I'd think (longer than has recently been supported). I don't see an immediate need to go 128 bit just yet, particularly for a workstation system (client edition).
Servers are another story, especially in the direction Intel wants to go with enterprise chips (i.e. 80 core Proof of Concept chip called Tera-Scale, and
Bangalore, a 48 core chip design just unveiled). Think clusters/cloud computing, which would need a massive amount of RAM for that many cores.

But not likely for home use.

Unless they've money to burn, and are rather nutz.
