Darn, well isn't this an interesting discussion. I'm still finding it really hard to hold back the urge to spend some more money replacing my 17-85 with the 24-70 f/2.8 USM ... Hmm.
I'd buy myself a 5D (yes, just a 5D; a 1D is just insane, because I don't need it...it's not like I'm running into blizzards or waltzing around the blazing heat of the Sahara). I'd get an L-bracket, a good gitzo or manfrotto tripod, whichever floats my boat (I haven't seriously considered a really good tripod yet). Probably a RRS ballhead with arca-swiss quick release. Lesse...lenses: EF 24mm f/1.4 USM L, EF 50mm f/1.4 USM (have it though), EF 85mm f/1.2 USM L, EF 135mm f/2 L USM, and the two lenses in my sig I'm fawning over. (24-70 for street stuff, the ultra-wide for landscapes and cityscapes) My 450D with the 55-250 is just fine for candids. 2 Alien Bee 1600Ks, just to have some *real* studio lights, and a couple thousand on props, props, props (backdrops, cheap cups, the props for a project I really want to do...hrm...I better get on that). And, er, that's it. Anything left I'd save.