You need a newer OS. 10.5.8 should work. I believe it is about 30 euro to upgrade. Apparently this is not the case ( thanks to Niel for the correction). It is possible that your computer may indeed be too old.
From the box the new iphone comes in and Apples website:
"Mac system requirements
Mac computer with USB 2.0 port
Mac OS X v10.5.8 or later
iTunes 9.2 or later (free download from www.itunes.com/download)
iTunes Store account
Internet access"
http://www.apple.com/iphone/specs.html
Message was edited by: roaminggnome