The OP doesn't state what version of SQL this is for, but as far as I recall, tempdb in RAM hasn't been an option since SQL 6.5.  SQL 2000 BOL states this about the tempdb in RAM option: "No longer supported because SQL Server 2000 has been optimized for maximum performance. "

As far as a separate drive for tempdb, I've had our production environment setup both with a dedicated drive for tempdb and having tempdb on the same partition as our data files.  We didn't see any significant difference either way.