What do you mean by high power gaming? If you're talking about Doom3 or Counterstrike Source, then you will need an Alienware or similar high (very high) end laptop. Most graphics cards in laptops can hardly keep up with desktop video cards from a year ago, if that.
My brother does plays America's Army just fine on his iBook, and you can get UT2004 as well. You won't have anywhere near the selection games though as you would on Windows. However if you buy a Mac, an XBox or PS2 would be better for gaming.
As for installing Windows programs, you would need to install VirtualPC to be able to run the Windows version. This will be slower, and is only worth it if you have one very specific program that doesn't have a Mac version. If you are serious about switching, you will need to factor in the cost of buying the Mac versions of your programs. Some companies will let you trade the Windows license for a Mac one for a low cost, but not many. You can get Mac versions of virtually every major graphics program and MS Office.
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