Hi Harry,
I have not put any tempdb directly into memory, but have been running tempdb in a separate cache for over a year now, with a very good performance
use master
go
sp_cacheconfig 'tempdb_cache', '48G'
go
sp_bindcache 'tempdb_cache', tempdb
go
sp_bindcache 'tempdb_cache', sa_tempdb
go
sp_bindcache 'tempdb_cache', sys_tempdb1
go
sp_bindcache 'tempdb_cache', sys_tempdb2
go
sp_bindcache 'tempdb_cache', sys_tempdb3
go
sp_bindcache 'tempdb_cache', sys_tempdb4
go
sp_cacheconfig
go
All the best
Mike of Sweden