Thanks
Thanks gpatpandp and atvusr. Ill let you know it goes.
From www.everymac.com about the maximum MacOS for the iMac 5,1: "This system can run the last version of OS X 10.7 Lion if upgraded to at least 2 GB of RAM. Although it has a 64-bit processor, it has a 32-bit EFI and is not capable of booting into 64-bit mode. It does not support OpenCL either. Please also note that OS X Lion is not capable of running Mac OS X apps originally written for the PowerPC processor as it does not support the Rosetta environment. This model cannot run versions of OS X newer than 10.7.5 Lion."
You can install 10.8 Mountain Lion either with MLPF or manually (Post #3654 and on). For 10.9 Mavericks there's a Guide in Post #914, use the Guide.txt in the linked "Mac-Pro-Xserve.7z" for the installation. If it's sometime finished, you can install Mavericks also with MacPostFactor. A alternative might be Parallels Desktop 9, see the Parallels Knowledgebase, Article-ID: 118806.
Since your iMac 5,1 has a unsupported graphics chipset, see also Post #1108 (quite the same for ATI X1600).
The App-Script 'SFOTT' (Sixty Four On Thirty Two) prepares a USB or disc install medium for unsupported 32bit based Macs that are not eligible for Mavericks or Mountain Lion. See the SFOTT webpage.
Thanks gpatpandp and atvusr. Ill let you know it goes.