i may have a solution for both your problems.
your games will lag because you have a AMD Sempron at 1.6ghz and 1gb ram. End of =).
As for the flickering desktop id say change the refresh rate of your moniter, "right click desktop, resolution, advanced settings, moniter."
If your refesh rate is on 60hz, change to 75hz and vice versa.
P.S. If you are able to overclock your cpu id highly recomend it, and do the same with your GPU, id also recomend installing atleast another 1gb of ram
AMD Phenom ii x6 1090t 3.8Ghz
8GB kingston hyperx @ 1066
Nvidia GTX 480 2GB Palit