Use Pascal Casing ( or Hungarian Naming after Charles Simonyi) like:
SalesOrderHeader, SalesOrderDetail, Product, ProductPhoto, ProductDocument
If you inherit a table with spaces in name, you will be married to [table name] (square brackets) for ever!
Underscore (_) still around in some system table names, kind of old-fashioned.
Kalman Toth, SQL Server 2008 Training