How big's TempDB now? unless you have a good reason, you probably want the total size of all the files to be about the same as the total size it is now with one file. So if TempDB is about 100GB now and you're splitting to 8 files, you want something around 12.5 GB per file. They should all be the same size.
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