I put the following in one camera bag (Tamrac 5606) and walked on to an Air Transat flight.
D300
18-200 VR
105mm f/2.8
50mm f/1.8
Sigma 10-20mm
300mm f/4.5
Canon S5IS
Shutter release cable
2 x batteries, 3 x CF cards, 1 x tripod head, 2 x glass filters, other bits and bobs.
1 x tub of acid for my girlfriend's contact lenses.
Gitzo 1325 tripod strapped on the outside.
The whole thing was about 12kg, over the 5kg maximum, but did fit in the size constraints. I was very polite, said that I didn't feel comfortable checking it in my soft backpack. The woman on the check-in desk said "that's fine." Just be polite and don't kick up a fuss/act like you're doing something wrong and you'll be fine.
As for while away, I know what file size my photos produce (measure this for yourself, depends on various settings. Your camera will]/b] lie to you if you're using JPEG or lossless compressed RAW as it expects the worst. Real-world you can get about 2x as many photos on as the camera thinks you can).
I know how many photos I can take per battery (again, measure this yourself. It depends how often you look at the screen, if you have autopreview turned on, if you use the screen a lot, brightness settings, how long your exposures are, if you have long exposure NR turned on, etc.)
I know roughly how many photos I take for a given scene or activity (which is again something only you can know.)
12GB of memory cards and 2 EN-EL3e batteries were good for 2 weeks in Canada where I took 1045 pictures, with a 50% split between 12bit RAW and 14bit RAW, lossless compressed. I charged both the batteries before I left and used both of them. I did recharge one of them once, but I could probably have *just* got away without doing so.
Obviously you have to work out these answers for yourself. Take 10GB of photos and see how many photos you got. Compressed photos have different sizes to each other, so the more photos you have the better an estimate for photos/card you have. Make them real world, a photo of a white sheet of paper will have a smaller file size than a busy, contrasty scene.
Your D300 has a pic meter in the battery info screen that tells you how many pictures you got since you last changed the battery. Make a note of the number every time you swap the battery out and you can work out how many shots you get per charge.
I don't know which lenses go with which cameras in your signature. Bear in mind that carrying the camera around all day might be a bit of a pain. I don't have a problem carrying 12kg of gear on a shoulder bag all day, personally, but I know that some people do.
Hammerhead... 1964 (in claret and blue no less)... Essex... Excellent taste in DSLRs (apart from the Sony of course)... You've got to be a West Ham fan?