not many boards support "registered" ram (at least not many boards for desktop computers) its primarily for use in servers
and i may be wrong, but i think your only option for a tbird or xp will be a board based on the amd 760mp chipset. that means it will be a dual cpu board (but they do work fine with a single cpu, you will just leave one cpu socket empty)
and any of the 760mp boards will support 1gb registered dimms in each dimm slot. besides obvious support for athlonMP, they also support tbirds and XP's
something like this board should work for you
link