It's all pretty relative. It depends on how much you made in 2007 and how much in taxes you have paid. It also depends on if you have certain deductions, such as child care and other deductable expenses. There are a few constants though in your situation. you can file "Head of Household" instead of just "single", which, in 2006, gave you a $7550 deduction instead of a $5150 deduction. Also, in 2006 you could deduct $3300 for yourself and anyone you claimed as a dependant. There is always a deduction for this, and it goes up every year to account for inflation. If your income is low enough (I believe it's around $30K with one child) you could qualify for the Earned Income Credit. You can also usually deduct $1000 for each qualifying child, as well as daycare expenses (you must have
a reciept.) Add this to any other deductions you may have (tuition, 401k contributions, etc.) and the possibilites for a refund are endless.
As a quick example, I didn't work for most of the year last year, and made roughly $17000. After all of my individual deductions ( I did not deduct daycare expenses because it is an in home babysitter) I recieved almost $5000 in a tax refund, most of that due to the Earned Income Credit. This year, I have been employed full time so I most likely will make too much for that particular credit.
The easiest way to get a 'ballpark" figure is to work up your taxes for 2007 on a 2006 form, using 2006 tables. The amount will be in the general area of what you can expect this year. give or take a few hundred dollars.