There's a name for this affliction: "GAS" (gear acquisition syndrome). I don't know anyone who's been bitten by the photography bug who hasn't suffered it at least occasionally.

I've seen two schools of thought around GAS. One school is, buy the gear that can do what you need for what you’re trying to do now. The other, is buy the gear you'll need eventually - and skip buying the intermediate 'steps'. Thom Hogan argues the latter convincingly (although he's talking about tripods, not glass or bodies). BTW, if you've got a Nikon camera, see if he's got a 'guide' for it. I buy one for every new camera body; lots more in there than in the manual that comes in the box.