I've got a Lowepro similar to this one -
http://www.dirtcheapcameras.com.au/s...?idProduct=355 It can carry my F80 with battery grip and 50mm lens attached, 70-300mm lens, 180 ED, heaps of film, spare batteries, filters, lens cleaning gear etc... I wouldn't want to carry anything bigger around.
Whatever your size choice, I'd reccomend a Lowepro for sure. They are designed and made very well, and they are very tough. Mine has seen some damp and dusty conditions over the last few years but it still looks like new, especially on the inside where the expensive bits are! In my opinion they're worth the extra $$$'s