How to use google voice with the iphone?

My wife just got the iphone for work. She has her normal cell phone. She wants to use the iphone as her main phone, her work is limiting her to 200 text messages per month, i know with google voice she can send free text messages, but i don't really know the best way to use google voice to solve my problem. Problem being: use the iphone as both her work and personal and still get around the text message thing. I hope that makes sense.|||Have her download the text free app, it works wonderfully.