Strange display at bootup

[magabyte]

I finally managed to boot into leopard, but now I get a strange display when leopard is finished booting(see attachment) the patterns are not always the same.

When I boot with -x the display is ok, but leopard is very slow and sound & lan don't work.

Is this because of my videocard or something else?
I have a ATI Radeon HD3850
i selected the ATI 38x0 driver package at install, also tried without but no difference

specs:
Asus A8NE
AMD Athlon 3500+
2 GB RAM
Realtek ALC655 @ nVIDIA nForce4
nVIDIA nForce4 LAN Controller (PHY: Realtek RTL8201CL/CP)