OCing the CPU is usually done by modifying BIOS settings. With OEM systems, like yours, those options are not available in the BIOS. If you had a non-OEM motherboard, you would be able to adjust CPU timings, voltages, etc, thus making the CPU run faster than intended.
As long as you don't enable OCing in the Nvidia Control Panel (for Nvidia GPUs) or in Catalyst (for ATI GPUs), you don't have to worry about anything OCing your GPU.
The recommended HD6570 will perform nicely at 1440x900.