In bowl, combine garlic, chili powder, cumin, onion powder, and salt. Pat mixture onto chicken breasts.
In nonstick skillet, heat 1 teaspoon oil over medium-high heat. Add onion and cook 1 minute. Add peppers and cook until vegetables are browned, about 5 minutes. Remove and keep warm. Add remaining oil and chicken to skillet. Cook 6 minutes. Turn chicken and continue cooking until cooked through, about 4 minutes.
Warm tortillas in microwave. Slice chicken and assemble fajitas. Serve with sour cream and salsa.