First you need a pci mac.Pci is the kind of card connector thats on the motherboard.If you don't know, you can check your manual.If you lost it,didn't get one or don't want to go digging around in the garage just go to www.everymac.com.They list basically every mac and more, so you shouldn't have a problem finding yours.Once you get to the page of your model, just look for Exp. Slots and if it says pci, your good to go.

After that you need to find a card.Go to http://lowendmac.com/usb/pci.html (low end macs usb card list) and find a card.There they have announced that they work with mac's.They also more or less have their own drivers.

What about all those cheap pc pci usb cards? Yes you can use those as long as they say they are Open Host Controller Interface (OHCI) complaint.So later on I'll tell you how to install the drivers for them.If your willing to go without official support, and want to save a few bucks then ohci is for you.I've seen some going for 20 bucks each!Thats a lot better then the 50 bucks for the official ones, but then you get support with that.

Each model (more or less) opens up differently.Some have screws,some the case pops out and some just open up (some G3 and g4's of course, you wouldn't be here if you had one of those).So I'll assume you have read your owners manual and know how to open up a case.If you are weary and don't want to install that card yourself you can ask a tech savy friend.If you don't have one, or don't want to void the warranty you can pay a shop like http://www.allmac.com/ (1800-we-fix-mac's) to put one in.