Well i would not even think of using an antivirus on a computer with that slow of a processor plus you are running a mac os operating system that is quite old, you should be fine, as for the browser I would go with safari, Those cant be that fast though, I'm a mac guy and I wouldnt even dare using those for the internet everyday they are too slow but that is my opinion.
So to recap:
1. no Antivirus needed
2. safari or opera which ever is faster.
There will be another opinion coming your way some time either in a few hours or tonight by our other resident mac poster. (i'm #2