David Car, if you want to use Greek under OSX and live anywere but Greece then you are in trouble! There is NO native support for Greek even if it says so, you have to install a patch called GRUpdate from Rainbow computer wich is the official Apple reseller for Greece. The problem is they won't give you the patch unless you buy a Mac with OSX from them - the force you to call and give them your mac id and then provide you with a password to get it. The GRUpdate provides you with a ton of new greek flags which you have to figure out which one works in a particular app. I attached a screenshot just to get you demotivated - 7 different Greek scripts that got installed somehow plus the 2 that came stock on Tiger - a total of 9 different greek scripts to be able to use all my fonts in all of my software.

Apple has promised full native Greek support under Leopard - we are waiting and running out of patience!!!!!
Now in your case if you just want to type Greek in Finder and not in any app like Photoshop, illustrator, freehand etc then i think you'll be ok, just don't use any third party greek fonts, stick with the Unicode ones that came stock.