YESYESYES ! Okay, finally I think I got it working!
Stable 125 FPS!
In the end I had to do the following to make it work:
Update to the newest ATI Drivers, now using "10-11_vista32_win7_32_dd_ccc_enu.exe" Used GPU-Z.0.4.9 to extract my VBIOS, edited it with "Radeon BIOS Editor, RBE", then replaced my old BIOS using "winflash20111" after making a backup of the original of course. After this the computer wouldn't boot correctly, when it came into Windows it took a few secounds, then the screen went black. I figured it was a bad flash or I gave the GPU to little voltage when editing the BIOS. So I went into Safe-Mode in order to make a boot-disc and re-flash from DOS. However one time while I was booting I forgot to set the default boot-device to my USB (which I had made into a DOS-bootable USB), so it tried to boot Windows. And it did. And it worked. No more black screen. Without me doing anything. So at my GPU Clock is set to 880 all the time. Quake still didn't work, but after I went into CCC, ATI Overdrive and UNCHECKED "Enable ATI Overdrive", it worked. Guess disabling ATI Overdrive ment CCC uses the BIOS settings instead, which I've set to my liking with RBE (880MHz in 2D AND 3D mode alike).
I also copied my Quake3 folder to my superduper fast primary hdd/partition with Windows on it, just to remove any obstacle it might encounter

Thanks for all the help guys, I noticed this is one of the longer threads here about this problem, so thanks for showing the interest.I'll report back if anything weird happens, but I think we finally got it.

edit: btw, the guys at ATI are frickin' idiots for not making it possible to disable PowerPlay from CCC. I'm not on a laptop, I don't need PP. I hate them and I hope they read this

Hah, just played my first online game since tweaking my configs to work perfectly. And it does. Almost forgot how much I love Q3
Btw, HYPER'DRUKQS, GG