I'm a FB fan of Dave Ramsey and the other day he came out with a budgeting aid. This is a general idea on how your income should be distributed - once you plug in your own numbers, you can see if you are over or under the accepted averages and where you can move money around:
charity 10%
savings 5%
housing 25% (Dave's not dumb, he knows many people live in locales where they spend more than 25% on housing, but if he prefers that you don't.)
utilities 5%
trans 10%
medical 7%
remainder is available for other stuff
(see:
http://www.daveramsey.com/specials/mytmmo-new-years-2012 You can plug in an approximate income to see the difference in numbers)
I can tell you from experiences with relatives that it is important to pay yourself first. They will always say that they can't find 5 or 10% to save, but then they run into emergencies and have nothing to fall back on (and as I've had to tell them, the First National Bank of the Cornfields is closed). The same goes for tithing or giving to charity. Do it, even if you are making those pennies squeal. You will still have the money you need.
Regarding the $320 for groceries. I buy for 2 adults and go back and forth between the USDA low cost or moderate budget (see example at:
http://www.cnpp.usda.gov/Publications/FoodPlans/2010/CostofFoodOct10.pdf). I could afford more, but I get pretty much whatever we want at $350-400 a month for groceries. It's doable.