Depending on when you did your installation, exactly what sub-version (B73.2 base, B73.2.1, B73.2.2) you are running, and what Service Pack, you might be able to get to SQL Server 2000. SQL Server 2005 is pretty much out of the question because the ODBC drivers are different.
Separating your database server and having a different application server is not a big deal. You can do that with the software you already posses.
However, your biggest problem is that no matter what you do, you will not be supported as all of those platforms have long since expired.
I suspect that you are no longer on maintenance with Oracle. If you are, then any move of this magnitude will probably have Oracle insisting that you upgrade OneWorld to something more recent. Only then would you be able to get official support for Windows 2003 and SQL Server 2005.
Dusting off the old memories, I seem to recall that an upgrade from the B73.2.x family up to the B73.3.x family (Xe or ERP8) required a lot of little workarounds which the manual didn't address. I'm a pack rat, so I may actually have the stuff put away in a box somewhere, but the Good Lord only knows how many other people in the country remember what this kind of upgrade entailed.