BkMaxima2004

I just recently bought my 2000 Maxima SE. I want to put some money into the cars performance, but i think there is something wrong with it first. It eats more gas then it sopposed to, everytime i press the gas just of the start it makes this weird low skeak, like something needs oiling or something. Then i had a check engine light on and the computer said that it were my cats, but when i went to the NISSAN DEALER to get it checked they said that the car only needed reprograming. My freinds have all sport cars, but they have the american muscle ones. They all said that my cats are ****ed up and i need to get new ones. If you guys can give me your thoughts on this i would appreciate it. Also if you know where i can get some decents cats for an ok price that would be great. Thank alot.