I prefer the "vesa" driver because the "radeon" driver generates a lot of visual artifacts I can't get rid fo (at least not with any of the instructions from the radeon driver website).
I don't want a refresh rate tool to run during and X session, I want X to start into the proper video mode to begin with.

If I leave just one device section and I set it to VESA, it doesn't work:
If I leave just one device and I set it to modesetting, it doesn't load the radeon xorg driver, X11.0.log shows "modeset" as driver (not vesa) and it works fine without the visual artifacts. This probably uses the framebuffer (?) but not the xorg radeon driver?
But still, it won't set the proper mode.

If I run xrandr under the X session, I can set the proper video mode on the fly.
But I can't make it start properly.

I know I can configure the X session startup to include the xrandr command but that is not the most elegant solution and it definitely does not work for runlevel 3.

Understood.
I did notice it.
I have tried both with and without, the result was the same : 1024x768 75Hz.

Unfortunately no, and this is a drawback for my consoles.
I can live with it...

Probably this is the reason why fbset does not work ...

Recap on what I've done so far:

= system uses radeondrmfb for consoles.
+ added kernel command line video=VGA-1:1024x768M@75 in lilo.conf
Now consoles use 75Hz refresh.

- fbset unable to change timings

= X11 using radeon driver works, but at 60hz and artifacts.
= X11 using modesetting driver works, but at 60Hz, without artifacts
+ X11, once running, can be set to 75Hz and my timings at runtime, with xrandr and the mode I configured in xorg.conf
- X11 does not start in any other refresh then 60Hz, even if X11.0.log says it uses my 75Hz modeline
- X11 at runlevel 3 works only at 60Hz, only after login, if user has customized his startup with xrandr, can I have the monitor in 75Hz.

Q1: is there any way to set my custom mode&timings for the console? (after boot)
Q2: is there any way to make X11 actually start in 75Hz using my modeline?

To use the vesa driver, you have to disable KMS (blacklist the radeon kernel module) and use a VGA console. But the vesa driver does not understand modelines and refresh rates, so you're back to 60 Hz then.

If the proper radeon driver gives you visual artefacts, then you should try the options from the radeon(4) manpage, they have been added as comments to your xorg.conf. Try them one by one. Another option is to install the AMD fglxrx proprietary video driver, it may have better support for you GPU. Or you can try upgrading to Slackware 14.1.

It may also be the case, that the visual artefacts are a result of your modeline, because it isn't valid to VESA standards. BTW: Why do you need a mode line generated by Powerstrip on a Windows machine? Care to explain?

Ok, it's my misunderstanding of a few details.
I was using "Driver modesetting" instead of "Diver radeon" but not "Driver vesa" for the instance when I had an image clear of artifacts.

I will try the settings in radeon(4) manpage for the artifacts. (there are artifacts in the console, so radeonfb, ie the kernel, is also responsible)
If not, I will try the fglxrx driver later.

The visual artifacts happen at 1024x768 60Hz (and all other combinations that can be displayed on my monitor, 640x480 to 1600x1200, when connected directly).
The visual artifacts happen also in the framebuffer console as well (every video mode).
The visual artifacts are much more dense in X11 with radeon driver, then in console, and make X11 almost unusable. The console is ok, they are very few. With modeset driver under X11, the artifacts are very rare, much less then in console.

My reason for using a custom modeline is the following:
As I said before, I use a KVM switch, which is capable of 1024x768 75Hz maximum.
If I have both systems configured with the same resolution, refresh and video timings, then switching between the two is almost immediate, the monitor (CRT) resyncs almost instantly and I see the other picture.

If the settings differ, then the monitor goes blank (as usual for video mode change) and then syncs to the new setting and light up again, all this taking 1.5 to 2 seconds.
It is very annoying and time wasting.

Because I have trouble getting X to start with the right modeline, I have serious doubts that if I install the AMD driver, the artifacts problem may be solved, but the modeline problem (which is my main focus right now) will persist.

Thank you.

First: You don't need custom modelines for your KVM, VESA invented standard timings for a reason. You just have to use the same standard timing on both computers and you won't get resyncs.

Modelines are a feature, that fell out of use a long time ago. Back in the 1990s they were required to support weird fixed-frequency CRTs and you got to fiddle around with xvidtune(1). Today we live in a LCD world and these LCDs only understand VESA timings, they usually don't sync anything else. Beside that on digital interfaces modelines don't make sense anyway, they're just transmitting data.

Still supported are custom resolutions using the CVT formula, because sometimes people need that for connecting TVs or projectors. But you want a special version of (standard) 1024x768 and that is most likely not gonna happen with current software. Even if you get close to a solution with some crazy workaround, it will break with one of the next updates and you start on square one again.

If your KMS Linux console shows artefacts, then there is a good chance that your graphics card is broken, most likely it is the video memory. Before troubleshooting first check that your hardware is in perfect order.

As I have said, I use a CRT and I will use it as long as it works. I know it has been outpeformed by today's flat displays, but my eyes tolerate it better (as contrary as it may sound).

I doubt the hardware is really broken (video card/memory) because windows XP, windows 7 (with AMD drivers) and Linux Slackware 13.37 with community drivers work perfectly. No artifacts.

I do need a custom modeline because the timings don't match. With the default 1024x768 75Hz mode (ie the one returned by gtf / cvt) switching with the KVM is the same as changing video mode, because the left/right, up/down and pixelclocks don't match.
And they don't match for sure, because the 1024x768 75Hz mode in linux is narrower and offset down and to the right on the screen. So regardless, I would still need repositioning.
You can undestand that without a KVM, I would do this once in the monitor menus, but with the KVM and frequent switches ...