Tripod is the only thing ever photographer absolutely needs, unless you shoot strictly photojournalism and are constantly moving.
I found buying things when I needed them was a way to build my gear up. I started with the 18-70 and 55-200 that came with my camera and have purchased new accersories as I need them. For instance I was gong to shoot a small wedding so I figured I needed an off camera flash
(sb-600), I got a gig doing real estate photography and felt 18mm was not wide enough so I bought a wider lens (sigma 10-20), I've recently been getting more into portrait photography and therefore got a 50mm prime. After a couple years with my D-50 I wanted more from my camera so I purchased a D-200, I started messing around with the commander mode and found I would love another flash for more dramatic lighting set-ups so I got an SB-800 (I know a 600 would do fine but it was an xmas gift from my girlfriend so I went for a more powerfull unit) I have my first band shoot coming up and we will be doing location shots of the band, due to this I may get another sb-600.
Anyway though I find getting things when they are necessitated works best for me. Yeah I could go out and buy a 300mm lens but honestly I have not found many times when I feel I need a longer lens. Shoot for at least a couple months and then go to the store.