Because most feline URIs are viral, antibiotics are really only useful if there is a secondary bacterial infection. A better idea would be to boost your cat's immune system and give her own body the strength to deal with the illness. For starters, what food does she eat? Diet plays a huge role in how well cats are able to fight off disease. Stress can also be a big factor.
There's an amino acid supplement you could add to her food in case it's a herpes virus that's responsible. L-lysine can be found at any pharmacy or health food store. Mix about 250-500mg twice a day into her wet food until symptoms subside.
More info for you on URIs: