Is cephalexin 500 mg capsules good for a infected tooth?

Yes: Yes. As long as you do not have an allergy to it or penicilin. One cap three times a day and call your dentist.

Possibly: It depends on why you need to be on antibiotics to begin with. Penicillin is usually the first choice but depending on your health history and allergies this may have been the best choice. Hopefully you are taking this because your dentist has prescribed it. Do not self medicate or use someone elses medication.

"Good for": Antibiotics do not cure infected teeth. They control spread of infection for as long as you take them, so you probably will require Root Canal or Extraction within the duration of the course of antibiotics. While Penicillin is the antibiotic of choice, there are reasons to substitute an alternative antibiotic. Please take exactly as prescribed and until all are taken. Do not self-medicate.

Infected tooth: See a dentist to get your infected tooth treated or extracted. An antibiotic will not cure an infected tooth. Penicillin is the typical antibiotic for infected tooth but others can be used when there are allergies