Snapshot isolation does not create a copy of the entire table in TempDB, so the rest of your question is moot, that's not how it works at all.
http://msdn.microsoft.com/en-us/library/tcbchxcb.aspx
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability