Please remember that VL 6.0 RC3 is late stage development/beta code. Problems are still to be expected.
Copy the /etc/xorg.conf from your VL 5.9 installation into VL 6.0 RC3 and see if that works.
Also, this post is in the wrong place for the developers to see it. It belongs in the RC3 bug thread in the Distro Development section of the forum. Can you please post there?
eMachines EL-1300G desktop, 1.6GHz AMD Athlon 2650e CPU, 4GB RAM, nVidia GeForce 6150 SE video
CentOS 6.5 (will try VL64-7.1 soon)
Toshiba Satellite A135-S4727, Intel Pentium T2080 / 1.73 GHz, 2GB RAM, Intel GMA 950
HP Mini 110 netbook, 1.6GHz Intel Atom CPU, 2GB RAM, Intel 950 video, VL 7.1