I've had my issues with long runs and 'VGA' cables but I've never had an issue staying at 100' or less, running at 1024X768.
Last year though, through a comedy of errors, I ended up running over 200' using premolded cable, and as a result I learned a few things.
Firstly, I've cut open plenty of these cables and I have never seen 5 coax. Always 3 coax with the remainder TP.
I'm pretty sure that on these long runs it's more an issue of skew on the sync than signal attenuation. The reason that I say this is that I had 2 identical runs going side by side all the way to two identical projectors, and one worked, and the other didn't. I put in amps at the source and 50' short of the projectors, and they never made much difference. Ultimately I tried break out cables so that I could go sync on green, and while I got a picture out of it, the box I was using to do it (Extron RGB109 IIRC) degraded the resolution too much, but it did work. I was lucky really, as it was only certain laptops that wouldn't work. I never spent the time to troubleshoot further, as the whole installation only lasted a year and now the building is being renovated.
My point is, that from now on, I'll run proper 5 coax cable whenever I have to go more than 100'. I do a lot of these types of installations, and I usually opt for the pre-molded stuff for a few reasons. It's cheap, it works, and the id bits remain intact. This last one is nothing to scoff at these days with laptops getting more difficult to coax a signal out of when they don't think there is something to send a signal to. The other thing to watch for is whether your particular amplifier strips the id bits out or not.
Cheers,