Thanx DepperDan,
fortunately I was only peeking into the whole thing (ATI+k2.6.7) out of curiosity and to find out if there might be any probs with those drivers/kernel versions in case a system-upgrade is immiment. So these incompatabilities do not bother me too much at the moment, especially as the standard setup (ATI3.7+k2.6.5) works perfectly.
As far as it goes for the fundamental windowing server: The test-boxes used were running under Mandrake 10 and Suse 9 (see detailed specs below) which both came with the XFree-server (recent Suse already comes with X.Org, me thinks).
But another box for XOrg-testing is already set up, so - even if it sounds peculiar - I'm sort of "hoping" and expecting to encounter some problems that can be solved in some joint effort

After all, XFree will be history in many future distros and so it's a good idea to peek into XOrg right away.
by
specs/test machines:
basic: AMD Athlon XP 2800, Asus A7N8X (box 1), Asus A7V600, VIA chipset with GBit-LAN (box 2), 512MB RAM, DAC960 SCSI-RAID, Adaptec 2940UW SCSI, Radeon 9600pro (box 1), Geforce 4MX (box 2)
storage subsystem: 4xSeagate SCSI 10k (30gig) drives as RAID 0+1 array as boot/system drive, Seagate 120GB IDE (data storage)
operating systems:
box 1 - Mandrake 10 Community running kernel 2.6.5 and KDE 3.2.2
box 2 - Suse 9 running kernel 2.6.5 and KDE 3.x (dunno exactly)