GameTap writes: "Choosing a Mac for gaming is much, much simpler than choosing a gaming PC-and certainly easier than constructing a gaming PC. Apple's line of Mac computers is small, with two notebook lines, three desktop lines, and that's it. This relatively closed system of hardware choices means that developers don't have to worry about optimizing their code for the near-infinite number of configurations we see on the PC side, so that's a good thing.
But there's a flip side too. Apple's lower-end machines (the Mac mini desktop machine and the MacBook) use Intel's GMA 950 integrated graphics chip, which shares 64MB of RAM shared with the main system RAM. And the entry-level iMac (the 20-inch 2.0GHz model, which starts at $1,199) uses an ATI Radeon HD 2400 XT with 128MB of GDDR3 memory.
Most games with high-end graphics require a dedicated graphics card with at least 256MB of RAM. So even though the Mac line is limited to just a few main hardware configurations, developers have to take the lower-end Mac's graphical capabilities into account when optimizing their game code. The result can be either diminished graphics on the Mac compared to a PC version of the same game, or a game with high system requirements that might exclude the Mac mini and MacBook altogether."