Hello and welcome to my User Tip
There is certain hardware and operating system requirements for 3D games on Mac's, just downloading or sticking the game disk in and installing isn't going to always work.
Mac's, unlike PC 3D gaming towers, one can't upgrade the graphics (some older MacPro's still can though) so one has to be careful to check the specs and find out what their hardware is capable of doing as 3D games evolve and require more recent graphics capabilities.
It's not uncommon for PC 3D gamers to upgrade their graphics card every few years to keep up with the latest games.
What determines a 3D game?
It's if you can move around almost freely and almost any direction in it's artificial environment much like you do in real life.
Some games are 2 dimensional, but drawn on the screen to look like 3D, or control the aspects or view points in some manner as not to have to draw other parts, thus can work on less powerful hardware. This occurs a lot on iPad and iPhone games.
Most 2D games don't have much of a issue on most Mac's, it's the 3D immersive ones that require huge performance in hardware.
Many 2D and 3D games show nice intro graphics or movies, but not the actual game itself so you can't make a quick determination, sometimes screenshots online and YouTube videos can assist to see the actual gameplay itself, not the movie parts etc., that often play in between maps or scenes.
1: Find out the hardware and OS X version requirements for the game(s) your interested in from the developers web site.
You may have to dig around a bit, and while doing so also find out how they update the game, if it's kept up with more recent OS X versions or do they cut it off at a certain OS X version and will they update it or not for the next OS X version.
Example is here.
2: Next what you do is determine the hardware you have or are intending to purchase.
For Mac's there is the free MacTracker here and it will tell you the graphics specifications of the Mac your interested in.
If you already own the machine, then open System Profiler in your Applications/Utilities folder and look under graphics to see what you have.
Newer Mac's with Intel processors have Intel HD 3000, 4000, or 5000 integrated with the processor graphics and are always well behind the current dedicated graphics from ATI or Nvidia.
Since all recent Mac's have Intel processors, they all have Intel HD graphics, but some Mac's have a extra more powerful dedicated graphics chip as well, these are more ideal for 3G games, but they can become obsolete fast as new games and improvements come to the games themselves.
The 13" MacBook Pro only has Intel HD graphics for instance.
3: Determine the Performance Rank of the graphics involved.
See if your machine meets or exceeds (better) the minimal specifications required of the developer.
To do that you use this web page and search for the card the developer mentions as the miminal
(note "M" means for Mobile/laptops, less powerful, don't confuse with the desktop graphics)
You get it's ranking then compare that ranking to your hardware and see if your hardware is lower.
Obviously a lower Rank is a faster graphics, so if your machine is faster than the specs required you shouldn't have much of a performance issue with the game.
However developers can and will stop making adjustments to the game in the form of update/patches so it works on the later video cards. Sometimes they can't, or are moving to a new game engine or other path (like falling sales volume) and thus will discontinue development on the game for later hardware.
So before you buy, you run the measures above and then you ask around if the game is slightly old (check Wikipedia and elsewhere for the release date) if they are still updating it or not.
Once you have your confirmation your good to go.