I've got a new Imac , but all the GOOD games are 20 years old, is it possible to play?

If you're looking to play old games, you can utilize either virtualization or emulation. You can emulate Mac OS 9 with Sheepshaver, which will allow you to play many of the above games. Alternatively, you can use virtualization software (Parallels, VMWare, Virtualbox) to install an old copy of Windows on it and run them without rebooting as they're hardly graphically intensive. In either case, it's quite possible. I wouldn't trust Crossover to run old titles like those.

Heck, you can get Myst on the iPhone these days.

You can, if you purchase and install CrossOver Games for Mac:

http://www.codeweavers.com/products/cxgames/

CrossOver is based on WINE, a popular Linux application that lets you run Windows applications on non-windows platforms.

You should also install Steam:

http://store.steampowered.com/

which lets you browse and purchase Mac compatible games. There isn't a whole bunch of games made for mac, but there are a few good / popular ones like Team Fortress 2 and Portal.

I'm not sure, but if your interest is in gaming, you made a wrong decision in choosing a Mac.