This usually depends. What kind of games are you planning on downloading?(I know some really good game console emulators for mac)
As for a pc emulator for mac... Virtual PC(Trial) runs on non-intel machines, or my favorite, Parallels that runs only on intel machines. I'm unsure if parallels has a trial or not... If you find a free pc emulator, you'll need a windows installation cd to run it.
Here's an emulator site:
http://www.macwindows.com/emulator.htmlVirtualBox is free and powerful but it still needs an operating system cd. If you choose this, download the free developer's version of Windows 7. That way you have a windows system you can play games on for free.
-Gandolf