Hi I am currently a linux user and I am quite happy with the os but was considering buying one of those mini macs as I cant afford one of those nice bigger machines that apple makes
Good for you. I bought a laptop from them and I love it.
#1 concern usability. I need a strong os and machine as i do Audio and movies personal and work related.
Macintosh is the premiere audio/video platform. The Mac Mini may be a little weak for that kind of stuff, though. Anything with a G5 will fare much better (the iMac is $1300, though).
#2 also i like to play the occasional game. Is their many games for mac?
The game selection numbers somewhere between Windows and Linux. And there's lots of simple/abandonware stuff out there that still works and is still fun.
#3 Security like unix/linux is anti virus need or dose it use the same root structure were a good firewall is only needed?
Mac OS is just as secure (or obscure, as some would argue) as Linux, so no AV is needed - although firewall is always good to have. It comes with one but it's turned off by default.