After a couple of hours compiling I have setup (emerge KDEbase) the KDE 3.3.2 on my Gentoo 2.6.11-r3 environment.

Unfortunately startx stops with an error message;
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(WW) Radeon: No matching Dveice sections for instance (BusID PCI:1:0:1) found

Fatal server error:
Cannot run in framebuffer mode. Please specify busIDs for all framebuffer devices.

XIO: fatal IO error 104 (Connection reset by peer) on X server ":0.0"
after 0 requests (0 known processed) with 0 events remaining.
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I am using "Radeon X300 series, 128Mb (1280x1024x75hz, 32bit)" adapter.

How to solve this? Please advise?

Ok. I have used xorgconfig to reconfigure it.

Video problem is gone for the moment. The error about video is gone by using standard VESA as video mode to prevent any problems. I want to sort that out later... since I want that KDE to work!

But now my mouse is not working and creating problems which prevent startx to launch KDE. I have tried in xorgconfig; 1 auto, 2 sysmouse, 3 mousesystems, 5 microsoft, 6 busmouse none of them work.

I'm using a USB optical mouse (windows is identifying it as USB-HID) but can't get it to work.

Again, please advise?

try using "modconf" to install the appropriate module (i.e driver) for your mouse.