Hi Becca,
Depending on how many miles the car has on it and the overall shape of the car would help make this an easier question to answer. According to kelly's blue book a 95 nissan altima SE Sedan 4Din with 100k miles in fair shape would trade in for $995 and private party sell for $2450.
If you could buy the parts needed.
I looked in the parts catalog above and they would range for:
Catalytic Converter runs from $131 - $171
Muffler $109
oxygen sensor $30 - $115
Nissan Altima Replacement parts so your looking at about $400-$700 with parts and labor, so if the car is in good shape it might be worth fixing it up. Since if you part the car unless you want to strip it down and see headlights and all piece by piece, its up to you. But I think thats a lot of time to spend on parting out a car. You could always see what a local slavage yard or junk yard would pay....