I strongly suggest you get a copy of Jerry Lodrigus' Beginner's Guide - here's the link http://astropix.com/BGDA/BGDA.HTM
. Jerry has written four ebooks and almost everything you need to know is there.
Now on your specific question, remember some things
1. The 17-40/4L has soft corners. This is generally bad for astrophotography. It sharpens up at f/8 to f/11 but this doesn't work either because every stop means you double your exposure (or since you're not on a tracking mount, the available light is halved.) Half the light means roughly half the stars - something that's not really helping.
Rather than the expensive 16-35L, you would do far better if you purchased a 35/1.4 prime lens and stopped it down to f/2 - f/2.8. This allows a much shorter exposure which will suppress star trailing. A 35 mm lens would allow around 7-14 seconds exposure. 7 seconds will give you a blur of no more that 3 pixels, irrespective of where you point the lens in the sky. You can always take lots of images and stack them into a very deep image.
This leads me to an often overlooked point - the sky does not appear to move at the same angular rate for all places. If you point a camera at the celestial equator, stars appear to move 15 arc-seconds per second. If you point it at the celestial pole, you will record movement in the corners and nothing at the centre of the field. This makes deconvolution quite difficult and it's worst for large fields of view.
2. Increasing the ISO doesn't really change the sensitivity of the CCD sensor - all that happens is you get more electrons out per photon in. This degrades the signal noise to photon noise ratio. There's a sweet spot at which the camera noise matches the photon noise and the overall noise is minimised. For the 5D MkII this is one third of a stop above ISO 800 or ISO 1600 (subject dependent.) That makes is ISO 1000 or ISO 2000. The third of a stop comes from the weird way Canon configured some of the signal amplifiers. I can't tell you the optimum for the Mk III but it's probably 1600 ISO or thereabouts - you can easily check this.