While that system would probably run STO on very low settings, please be aware of the operating system mac uses. PC uses Windows, mac uses OS X. Due to the differences between OS X and windows a game must specifically be designed for OS X to work.
It seems Cryptic are currently beta testing an OS X version of the game.
As you can see from that post your system is right on the minimum requirements for your Graphics card and you have plenty of RAM. With your quad processor being above minimum spec. Most likely it will run but like I said, low settings.
You'll want to update the graphics, but everything else should run it fine...
This. Is it a Mac Pro, or a flat-screen iMac? If it's a Mac Pro, you can use a variety of AMD video cards (and a rather small selection of Nvidia cards), but if it's an iMac, you probably can't put a new video card in it.