Stimulant Effect of Green Tea

Will taking a herbal capsule containing 200 mg of green tea extract cause a noticable stimulant effect on someone not used to caffeine?

It is possible, but we suspect that it is unlikely. But everything depends on how the extract was made. You should contact the manufacturer directly and ask how much caffeine is present per capsule. A cup of coffee contains about 100 milligrams of caffeine, so I would say any amount over 10 milligrams might be noticed by someone not used to ingesting caffeine.
For more information, see the caffeine information web page put up by the McKinley Health Center of the University of Illinois: www.mckinley.uiuc.edu/health-info/drug-alc/caffeine.html.