You should not be seeing slowdowns at that resolution with your hardware. Can you check system temperatures, and also confirm that both CPU and GPU ramp up to full speed when gaming? (CPU-Z, GPU-Z, and CPUID HWMonitor are helpful tools for analysing your system, in case you don't have them already).
Also, what power supply do you have? If it is of low quality or insufficiently powerful, your performance will suffer.
Oh, but I recently found out that although I have closed some of my pc programs that are not needed in MSCONFIG, when I go to the desktop and not open any program, it still consume 1.18GB RAM Of the 4GB RAM from the performance of the task manager, will this cause my RAM become not enough for recent and upcoming games? Thx