This is quite an interesting KB article detailing I/O requirements for tempdb, and amongst other things, the use of RAM disks and solid state media.  There are some important points to consider e.g. as the media is not durable, only tempdb can be located on it; it cannot be used for other database files.  It is also worth remembering that the performance of temp tables will not be great due to the double RAM caching mechanism.   I recommend reading the KB article for the full details http://support.microsoft.com/kb/917047 Microsoft SQL Server I/O subsystem requirements for the tempdb database