Picking a Camera for Model Photography...

I have a Sigma SD1 that I love and get along with very well. If I shoot my usual non-moving subjects, I can get great results. I don't do bad in good light shooting mobile subjects, but often get a little restrained by the processing speed. More recently I have been shooting concerts, dancers (e.g., belly dancers on a dimly-lit stage), models in lower or natural light, and sports photography. Things that will benefit from more leeway in the ISO, fps, and processing speed. I think I need a second camera to better handle those situations.

I am not beyond outfitting a new setup, probably in the Canon or Nikon line, but I also have not really been following those lines too closely. I'd like some input from those who own the equipment. [I know, I know... go to the store and hold one, etc. I will...but I am hoping for an elimination round...] Let me give some background...

I like my Sigma because:

* I like the resolution. No arguments about the 15.4 MP.

* I like price-sensible package. The SD1 ($2000), 3 flash heads, 70-200 2.8 EX ($1000), 17-50 2.8 EX ($600), 105 2.8 EX ($750), and more rounded out to less than 5K.

* I like the easy conversion to IR. Pop out the lens and remove the dust guard and ready to shoot IR.

* I like the handling and feel.

* It does the job for my still photography.

For the purposes of lower light/non-still photography, I don't like:

* the noise at higher ISO. I rarely shoot at anything other than 100 ISO because I feel there is significant noise.

* the slow processing speed. Once I stack 7 shots, I'm waiting behind the viewfinder...

* the limited fps. Tied to the processing speed... I can get nice bracketed bursts, but often have to wait between.

What am I hoping for:

* I am D4/EOS 1DX tempted, but I think I am probably better off in the short term making sure I have enough difference in performance in accord with what I am looking for to justify going that way. I'm probably looking below the top-of-the-line, unless that is my only choice. I'm leaving the budget open.

* I know a lot of it will depend on good glass, and I expect I'll be shelling out another $3-5k for 2-3 nice lenses initially.

* I want the ability to shoot a lot of frames without being held up by the camera. Models don't need to hold a pose... Hurling and Hockey fly by...

* I want to maintain resolution and depth. I do a lot of photo-adjustment, and shoot in RAW.

* ISO flexibility will help a lot. It doesn't need to be extreme...but picking up some stop leeway without heavy noise is a boon.

* This camera can be dedicated to motion/sports and low/natural light... people photography mostly.

I'd Be happy to hear...

* Your suggestions for specific cameras, and why.

THANKS for your input.