it is invariably going to be a power issue if its not the bios.
i get what others are saying, "400w is enough" etc.
but that does not account for the amperage per rail, or the stability/cleanness of that voltage.
Are you using pcie connects from the power unit to the gpu? if youre using the 2x molex to pcie adapters, then you may want to switch the 12v rail to another if it has another, ideally in that situation you would connect one of each molex to seperate 12v rails to balance the load, if youre connecting two of the same molex pins from a single cable then youre going to run in to problems on the cheap psus those systems ship with.
If its the bios and youre running the Gateway DX4380, then you need to download this.
[EDIT] FOR DX4870 SYSTEMS!
Note: Download either the Windows 8 UEFI bios or the standard bios, depending on what your computer came with, if it had windows 8 installed from new, download the windows 8 bios, if it was windows 7 or older, download the windows 7 or earlier bios file.
Windows 7 or earlier BIOS FILE : http://global-download.gateway.com/GDFiles/BIOS/BIOS/BIOS_Gateway_P01.A2_A_A.zip?acerid=635040993388166881&Step1=DESKTOP&Step2=DX%20SERIES&Step3=DX4870&OS=ALL&LC=en&BC=GATEWAY&SC=PA_6G
Windows 8 BIOS FILE : http://global-download.gateway.com/GDFiles/BIOS/BIOS/BIOS_Gateway_P11.B1_A_A.zip?acerid=635321007306718972&Step1=DESKTOP&Step2=DX%20SERIES&Step3=DX4870&OS=ALL&LC=en&BC=GATEWAY&SC=PA_6G
AMI ROM Tool : http://www.ami.com/Support/downloadagreement.cfm?DLFile=support/downloads/amiflash.zip&InpDrvID=90
Remove the new graphics card and boot into windows, extract the AMI ROM Tool called "afuwin64.zip" from the zip directory amiflashAptioafuwin64 then extract the folder AfuWin64 from this zip and run AFUWINGUI.EXE
In the software, click "Save" and save the current bios somewhere to your computer, just incase you need to restore it to the current bios in the future.
Click "Open" and select the .cap bios file in the bios you downloaded (in the ROM folder), once done click "Flash" and let it update and reboot to windows again, shut down as normal then install your new card.
HOPEFULLY, you should be good to go.
