Part of the answer lies in what version of SBS 2k you have.

Did SBS 2k come pre-installed on your existing server or did you purchase Open License version or a full retail version

If SBS 2k came pre-installed you cannot move that copy to new hardware.

If you were seriously considering dual Quads, save some money, buy dual dual cores and also migrate to SBS 2003 with software assurance.  then when the next version comes out next year, you'll already have 64 bit compatible processors

I need to get a new server for my SBS2K Will SBS2K support
2 Quad Core Multi-processors?
or 2 Dual Core?