The interchange panniers i have (discontinued) can carry about 4 bags of groceries between the two, maybe more depending on which items i get & their size/shape. I can do a month's worth of groceries minus bread (too bulky) in one trip. As soon as i find out the best way to attach my milk crates, i'll be carrying cases (24 pack) of spring water (separately, no other load). If i'm not carrying water, i may be able to fit the bread in there.

Plan B (for Bread and for Backup) is to get a pair of those wald wire folding baskets specifically to carry all the bread. Since the primary grocery list is checked off at a 24 hour store and the bread is at one that isn't 24 hours, i'd be making two trips anyways, so hooking up a pair of baskets for one short trip is no biggie.

If you're still looking, i would definitely recommend you at least look at these:
http://www.bontrager.com/model/11573





- Andy