My current employer made just such a jump. Their ERP system was on an AS/400 box running Oracle, so was their GIS and Permits system (the latter two on Dell servers). The ERP was a horrible system that was designed for universities, and poorly at that. It's now on SQL Server, as is the GIS. The Permits is in Access and we're in the process of migrating it into our ERP system, though I'll be converting it and the old Oracle data for it into a SQL Server DB for historic lookup purposes.

The cool thing about our Permits conversion is that we're not converting data! All new data goes into ERP, all old data will be closed out in Access. WHEE! Happy day! That's going to save so much hassle!