70-300 or the 75-300? I never thought of the 70-300 as a Kit Lens. It is soft-ish from 200-300 and a mkiii isn't going to change that at all. I've said this a few times now, but why don't you see if you can your hands on a t2i, t3i, t4i, t5i, 60D, or 7D because they have the same sensor AND SHOOT IN RAW!!!
Compare the iso values (maybe look up an article or post here about native iso's), and see if you find it tolerable. I'd also suggest tripod mounting the camera to eliminate shake as a variable.
I personally would prefer to get a while host of lenses to go along with my $800 used 7D and spend the saved money, 2200 on really nice lenses. Maybe the 300mm f/4L, or the 70-200 f/2.8L USM plus a 1.4x teleconvertor, a 24-105 would be a nice little upgrade over the 17-40 provided you don't love the 17-24mm... though for some/many 24mm x 1.6 is a touch too long and not wide enough.
I guess the real question is... how much money do you have? Can we have some?
Everyone's definition of acceptable noise is different. It matters what your definition of acceptable noise is and you won't find that out without getting a body in hand. With lightroom, you should easily be able to shoot at 3200 with the 7D and still come away with some shots that look great viewed on faceboook. I forget what the article I read said, but raw images have significantly more information that has been captured, so it is easier to change the exposure and to bring out the colors that were originally there. It does take effort to do the post production, but you have to ask yourself whether you want your shot to look like what the camera sees, or you want it to look like what you see. And that is often a big difference.
I'd suggest shooting in both raw and .jpg for a while... and then really GOOD ones... do some post production with and see if it is better than the .jpg.
The more light, the better the AF... more light comes from the larger aperture of the lens... larger aperture lenses usually have better motors which are faster...
How would a 7D + 300mm f/4 combo go?
Part of the reason I suggest people buy used but in good condition is because bodies depreciate quickly. @ $800, you can probably sell the 7D in 2 years for $700 or maybe $600. So the real cost to own the body is only $75ish per year. If you buy new, you are easily looking at a $400 depreciation after you take your first shot.
I paid around $2600 for my new 5d mkiii (I got a really good deal for a new one from an authorized manufacturer). In 3 years right before the mkiv comes out, I can probably sell it for $2000. After 7 years (around the time of the mkv), I'm maybe looking at $1400. I'm guessing at the timeframe of the new generations and the money I can get, but I'm paying about $171 to $200 a year provided it remains in good condition.
It is hard to know exactly what will be best for you, but I'm the kind of guy who likes to minimize risk... so if you get an $800 body and another $2200+ in lenses, you'll be better off than if you got a $3000 body and stayed with the 70-300 (provided it is a 70-300).
The 70-300 is about 100x better than the 75-300... and the reason I want a clarification is because very few people would suggest going out with a 75-300 and a mkiii with the expectation of getting good results.