Installing OS X 10.5 on an iMac running OS X 10.4.11 - won't work?

I'm having trouble installing Leopard (10.5, I know, I am a little late with this). I'm currently running version 10.4.11 of Tiger. I've inserted the installation disc (it says "CPU Drop-in DVD"), restarted the computer and chosen the language - then the progress stops and it says that I cannot install Leopard because I don't have OS X 10.4 installed. Seriously, what? I've tried a few times and everytime this message comes up.

I have an iMac with 2GHz Inter Core 2 Duo, and a memory of 1 Gb 667 MHz DDR2 SDRAM. I don't know that much about computers but I find this very odd. I've googled this and searched Apple Support but haven't found anything helpful, so I decided it's time to ask you guys!

Did that disk come with your iMac? If not, you'll need to buy the full retail copy of Leopard as disks provided with computers are machine specific.

It came with my iMac, I have no idea though why I haven't tried to install it before..

It sounds like the Leopard DVD you have is the upgrade version or "drop in" version which will upgrade Tiger to Leopard. Did it come with your Tiger DVD set in the same package? If it is, it should work, however, I don't know that it's meant to be used after you have installed all the updates for Tiger. I believe it was supposed to be used when you first turned on your machine.

Why don't you just forget it and update to Snow Leopard? As long as you have an Intel machine you can use the SL upgrade disk. $29 at the Apple store or Best Buy.

Well I suppose that I could just buy Snow Leopard since it seems to be quite cheap, I just need to get OS X 10.5.8 or newer so that I can use the iPhone 4 I got a while ago. It kinda sucks that it won't work with Tiger but maybe it's time to upgrade the system anyway
Thanks for the responses!

Yes go with Snow Leopard. Tiger is an old operating system no longer supported by Apple, and with the upcoming release of Lion, Leopard may well go the same way.