How much of a mortgage should i go for?

I currently earn £18k in a sales job, bonuses take it up to about £22k. i can qualify for a graduate mortgage and am looking at properties around the £80k mark. i have loans and credit card payments of £170 per month. what should i go for?
Answer
I would go on the halifax website and look at the mortgage calculator and you can also chat to an advisor online too.