PC user: Macs are rubbish, why use them?
You: why are Macs rubbish? They can play games just like your PC, they can browse the internet, email, and basically everything your PC can do. And it usually can do it better. Macs don't get virii or spyware.
I ask many of my friends why they say 'Macs suck' and every time, they give me some spin of answer about clock cycles or the latest games.
Basically, if you want a good comeback, learn the facts. Hang our around the PC and Mac hardware and software websites, see what they have to say, collect information, and use it.
http://www.tomshardware.com is good place to start for PC hardware.
And remember, clock cycles are next to useless in trying to compare an x86 CPU against a PPC.