my wife has been a student for the last four years...she is now 25 and have never filed IRS income tax....she now work well over 29000 per year....she started working since may 11, 2007..after graduating for University of Texas.....will she have to have to get a joint sponsor?

Technically, she should be fine as current income is more important than past income. However, they also look at how stable her current income is and could decide that as she's only recently started working it's not enough. If you are able to get a joint sponsor, I would do it to be safe. Also, your wife will need to show that she didn't have to file a tax return yet - a statement explaining her student status in 2004 and 2005, and citing the IRS regulations that say she doesn't have to file unless she makes a certain amount (I'm sorry I don't know the specific section though).