6-core Xeons or Core i7s with XP will require the hotfix https://support.microsoft.com/en-us/kb/958244 to be installed for it to reboot properly.

^Note taken. Disabling HT would also be an option (well, up to an octa-core anyway).

As for X58 suggestions--well, while I'm a fan of this beast of a platform, however at this moment I'd say dedicating an X58 build for 32-bit XP doesn't seem very economical (if you care about that kind of thing at all, sheesh 😜). Especially when the Core 2s are already "cheap as chips" as some say, AND also the better LGA775 boards aren't as expensive as the X58 ones. I especially like the later gen X48 or P45 with DDR3. What's 'meh' about them are only if you decide you want to do SLI, as nForce was still a thing then... In that case X58 SLI boards would be the better option. [edit: I'm open to anyone sharing their experience on X48 + SLI, might do another thread]

Buuut since you have the Radeon pair, then Crossfire would be fine with boards such as ASUS P5E3 Pro (I don't have a list of viable models though), or go all-AMD build, which could be an even better value.

NB: The X58/Xeon combo is such a godsend for people building budget Xeon workstations + Quadro (e.g used 4000/Fermi 2GB comes for less than $200 and is still a great choice for CAD). RAM can be upped to 48GB (6x 8GB despite 24GB max on spec -- you'll find examples on the net). But don't help spread the words please, I can't stand the used X58 prices, especially the Rampage series and the likes. 😐