Sensitivites. Hopefully, if you were sick enough to need IV antibiotics, cultures were done to see which specific bacteria caused your cellulitis. If so, then the lab also tests to see which drugs the bacteria is "sensitive" to. These are the best ones to use. These can vary depending on the type of bacteria and where the infection is, if you caught it in hospital or at home, etc. As always, finish all the med!
Depends. Usually tailored to the specific case, your medical history, severity, allergies, any cultures etc, but most commonly bacteria such as strep and staph i.E gram positive bacteria are the cause, so penicillins, sulfa drugs like bactrim, (sulfamethoxazole and trimethoprim) cephalosporins would be good.