You can probably go into the BIOS and disable the onboard ATI GPU. Then make sure you uninstall all the ATI drivers (Use Driver Sweeper) and that should remove any conflict potential.
And Sleep Mode by itself is a joke, so don't use that to begin with. LOL
As for drivers, ALWAYS install the latest nVidia drivers by downloading from their website. I never install the drivers on the disk with the GPU. Considering it's a 9800GT, those drivers are probably 2 year old. LMAO
A non-branded PSU in a PC is never a great idea. It's like choosing the generic brand Pace-Maker after having a heart attack. If you consider that the motherboard is the computer's brain, the PSU is the computer's heart. With a bad heart, nothing works right, and the potential for catastrophic failure is much greater.