You will always work faster on a system that you are comfortable with. The same is true for software packages, especially commercial apps like Adobe stuff.
For occasional use you replace Photoshop with Gimp, Illustrator with Inkscape and InDesign with quite nothing else on the open source front - but you will not feel very lucky. For pure graphic design stick with the Mac.
However, if you want to do advanced web design stuff with a lot scripting, then it makes sense to set up an additional Linux box as a server. It is especially useful if you want to test web appearances for non-Safari and non-Mac systems. Of course, you also need a Windows box to do a complete test range.
In your case the problem is not really if Linux can compete with a Mac, but rather that Adobe is avoiding to port apps to Linux. They had once Photoshop ported to SGI Unix, but then abandoned UNIX altogether.
It is possible to run Adobe apps on Linux in emulation via wine (works quite acceptable) for Windows versions of those apps, but since you have the Mac versions, this option will not help you much.
If you have some old mac68k type of Macintosh computers around, then you can install BasiliskII and run all those Mac applications emulated. It's quite nice to see an old Photoshop 2.5 running at about 500 times the speed vs. the speed it did on those old Mac hardwares (like the Quadra top line featured on Jurrassic Park 1. as the "top computer").
Last edited by Tinkster; 10-30-2010 at 03:25 PM.