It depends on whether the kitty has obvious signs of infection (green eye goop, for example) or whether he/she is just sneezing. A lot of cats have a form of the Herpes virus that gives them recurring colds. Sometimes Lysine or Interferon can help them get over the cold faster. For cats with bacterial infections, antibiotics (either oral or topical) are often provided.
Clavamox to start, then Zithromax or Doxycycline if that doesn't work. Lysine every day for maintenance.
The newest research indicates that many URIs are caused by mycoplasma, not bacteria or viruses. Zithromax and Doxycycline are both effective against mycoplasma as well as bacteria so they pretty much cover all your bases. I just use Clavamox first because it has less risk of side effects.