Your best bet is to start taking pictures with the camera you have, just to see if you have fun shooting still pictures of aircraft instead of filming aircraft with video. If you enjoy taking stills of aircraft then by all means make the investment in professional equipment that will give you better results than you will get with the camera you presently own.
If you have done some basic research you will come to realise that the optics or glass of the lens is the most important investment you will make. A good zoom lens 200mm or more, with a low fstop, i.e. f2.8 equals quality, but also means expensive as well. Nikon and Canon both make very good lenses, you can't go wrong with either. The choice of lens manufacturer will determine which body manufacturer you go with.
I use a Canon 60D Body, with a Canon f2.8, 200mm Prime Lens (Very Sharp Lens!), a Canon 70-200mm f4 zoom lens, and a Canon 100-400mm zoom lens f4.5/5.6 zoom lens.
I once operated video cameras in the news gathering arena, before buying a DSLR and found the transition to stills harder than I thought it would have been, especially photographing aircraft, given my background and had to adjust my technique accordingly, so factor this when taking photo's of aircraft to start with.
Also when browsing photo's on airliners.net, see if any of the pictures that have caught your eye includes the type of lens and camera body used and take note of it, provided the photographer has listed those details in the comments section.
I also should mention that Photo Editing software and knowledge how to use it, is used to tweak photos, so what you see isn't necessarily what you get when taking a photo, but a good photographer, with good equipment wont need to edit much to begin with.
[Edited 2013-02-17 15:29:53]