I would definately get the hardware issues fixed first. If you don’t trust the hardware that it is on replace it and get it up to snuff first. then you can do the upgrade to 2003 after the HW issues are fixed. BDC/PDC roles went away in windows 2000 server. you would need to check to see if any FSMO roles. To find this out check this article:
<a href=”http://support.microsoft.com/kb/255690″>How to view and transfer FSMO roles in the graphical user interface</a>
if this server only acts as as a DC you can transfer the roles and theoretically down the server. but if this server has other roles, (file/print services, etc.) you would need to move these as well. if there are file shares it would be easier to look into DFS (distributed file service) so that the drive mappings are only a link in the dfs and you can replicate or move the files anywhere without the users knowing the difference.
You need to get a 2nd domain controller online as soon as possible. With AD, there is no PDC/BDC, domain controllers are all at the same level.
Build a new server, DCPROMO it to a DC. You may need to extend the schema of your AD, the DCPROMO will let you know if you need to do an ADPREP. Then transfer all the FSMO roles to it from the old server. I would then build a 2nd server if resources are available and DCPROMO it as well. Once everything settles out, you can DCPROMO the old server and repurpose it if it still addresses some need in your workplace. But PIIIs have limited value when looking at entry level server class machines on the market.