how to use onboard VGA with CUDA 4.0 RC?

I want to use CUDA 4.0 RC in RHEL 6.0 on a Tyan S7025 motherboard that has an Aspeed AST2050
onboard VGA capable of up to 1600x1200.
However, after I installed the devdriver for CUDA 4.0 RC, I cannot get the
windows-based environment to work with either init 5 or by rebooting RHEL 6.0.

How can I do the setup, so that the onboard VGA will be used to display in max resolution
of 1600x1200, while the 4 PCI-E slots will have c2070 cards?

Thank you!