I read an interesting article in the Feb 20, 2006 BusinessWeek on Donating Clothes, Books or Other Tax Deductions.  The article stated that case law requires all noncash items to be used for tax deductions need to be able to be backed up if they are over $250. 

There is a useful guide here Salvation Army.  Also, it is a good idea to print off similar items from places like ebay to validate the monetary value.