Upload & Sell: On
If you'reIf you're shooting on a 7D, from a 40D, and want to jump all the way to a 1D4 or 1DX, sounds like money isn't really an issue for what you're looking to achieve. And if you are doing lower light work, then yes, 2.8 will be your friend compared to 4 or 5.6.
A 1D4 is an excellent camera, and great bang for buck to be on a pro body, 1.3x crop for added reach, and a 300 2.8. The 1DX is an incredible camera, but $1500 more in some cases, and you could put that towards a 300 right now. Sell your 7D if you need or want, or keep your 70-200 on that, and your new 1D4 with the 300 glued to it.
I wouldn't get a 1DX unless $ isn't an issue, which it seems it might be if you have to wait on the 300.
Of course, you could get a 5D3, which is similar performance to a 1D4, though I think the 1D you'll gain a bit with the 1.3x crop, and the pro body, which on a 300 2.8 could help balance things on a monopod, etc.