I'm kind of in the same boat. Right now I have my kit lens (18-55mm) and I got a 50mm f1.8 for Christmas. I'm in between "wants" and "needs" right now. Alright, lets face it...I don't really "need" anything else but I think I need something with more zoom. I was thinking about a cheap 75-300mm but don't like the idea of losing the wide end. For the price it's not a bad buy though. What I really want though is Canon's 24-70 f2.8 L. I've been looking at it for a while but yesterday DW, DS and I had some professional pictures taken and guess what lens they were using? I think I may have sold DW on it...until she see's the price! If I really had it my way I'd be buying the 24-70 AND the 70-300 f.28 L but that's not going to happen.
I do like the looks of that 18-200mm Sigma but something with a little faster would be nice. I suppose that's where the $$$ come in.