PLEASE help me decide!!?

ok people I need to help on deciding what to do!! I work in a local SPCA and have fallen in love with this little long haired Chihuahua. The problem is that I live with my mom and we have 2 cats and a dog already. The deal is that if I get the dog I have to get rid of my cat that I have had since he was a baby kitten. I think I may be willing to do this but its a hard choice to make. I want this little dog soooooooooooooooooooo bad but I want to make sure that sam has a good home because he is so loving and craves attention. What would you guys do?? Gizmo has stolen my heart!