Does work experience help in (US) graduate admissions? I have around 8 years of work experience. At this stage, the possible step is to go along management track. But, I am neither interested nor does it appeal to me. Thinking more on this, I have decided to go back to school, preferably for a PhD or, in the worst case, a master's degree.

My GRE and GPA are not so strong: 1100 GRE with 770 in math and 3.0 GPA (out of 4.0).
My experience includes companies like Amazon and Zynga, but I have no research experience whatsoever. 

Am I a good candidate for PhD or master's with research? My primary interests are in computer graphics or applied mathematics in simulation/visualization. What do you  suggest I do? I am aiming for top 20 schools, but not sure if it will be right thing to do.
I would strongly recommend sitting in on, or somehow actually enrolling in, some upper level class at a local university. Treat it as if you were actually enrolled, and ask (beg) the professor to grade your homework and exams. And be sure to sign up for something brutally hard.

This will let you test your chops, and if you excel, then you can ask the professor to write a letter of recommendation.

Please keep in mind that top 20 programs are extremely competitive, and reject very many good people. Your competitors will have letters of recommendation from professors specifically familiar with the requirements of Top 20 programs, confidently saying that they are prepared to handle it.

An alternative would be to try to get into a master's program at a lower-tier university. If you prove yourself to be head and shoulders above everyone else, you can ask your professors to support transfer applications to better programs.

Your work experience is likely to count for very little, or perhaps even against you, since you have been out of school for so long. Sorry -- that's just how it is.

This is a hard road, I have at least sort of walked it myself. Good luck to you!