G means no aperture ring, D means distance information.
Just need to make sure you have AF-S or AF-I on the lens. You can't autofocus the screw driven lenses. No big deal really unless you have a boatload of high end older lenses or you want to use the 10.5 fish.
Regarding lenses...I think you are wasting money going from a 18-55 to the 18-70. You only gain marginal reach and are still stuck shooting a max F3.5 lens.
In my opinion, you should really only have one consumer level lens and that is the 18-200 VR (well maybe the 1.8 50mm as well). After that, build up your high end zoom trinity (DX=12-24 F4, 17-55 2.8, 70-200VR 2.8; FX=14-24 F2.8, 24-70 F2.8, 70-200VR F2.8) Somewhere in the process get your high end specialty lenses (think Macro, fish, tilt shift, prime specialty (eg. portrait, wildlife tele) etc...)
I have never regretted getting my good glass. Next on my list is the 14-24 (hopefully this summer) and the day they come out with a full frame fish I will buy it as well.
Anyways, just trying to keep you from having a bunch of useless lenses once you upgrade