I have an old Shuttle barebones SN21G5 (AMD 64), and I really need a more modern chip in there.
Is there a way of swapping out the old mainboard and putting in one of those new fangled phenom mainboards in there?
Shuttle has their own form factor which you can only get from Shuttle themselves. I recommend skipping Shuttle altogether and sticking with a case where they don't make up their own form factor standards.
If you still want to keep the Shuttle computer and switch out the motherboard, you need to contact a Shuttle reseller for information on what your options are.