Have you tested your DRAM with Memtest? Even when run at stock, one can have a poor module and Memtest86+ will inform you on your DRAM's operating stability. Download the Memtest Image "ISO" http://www.memtest.org/#downiso
and burn the ISO image to a CD. Reboot with your Optical drive being the boot and let the program run a full test run.
Please fill your system specifications with a more detailed listing. It is hard to trouble shoot when one does is not advised to all of the systems peripheral components.
One of the first decisions I make when building a new system is the power behind the system. These Rosewill PSU's are not stable with regards to ripple when compared to ones such as PC Power and Cooling, FSP, Akasa, Seasonic, etc.
The X1900XT is not only a hungry demanding beast. It is a non-tolerant one with regards to power being supplied strongly linear. If your driver installation is fine and you are not overclocking the video card and processor, then you have eliminated many variables.