What's considerd a low grade fever? and what's considerd a high fever in adults?

A temp of: 104 or greater would warrant medical attention. Temp >107.6 °F (42 °C) creates serious risk for brain damage. What constitutes low fever is adult is debatable. Check this med line plus article: http://www.nlm.nih.gov/medlineplus/ency/article/003090.htm

Fever: Generally, low-grade fevers range from about 100 F-101 F while high-grade fevers range from about 103 F-104 F. This is for adults. Your own "normal" (if it differs from 98.6 F might make it a bit different for you. Best!

See below: low grade fever is from 98.6 too 100.4. A high fever is above 102 degrees F