Upgrade my iMac 10.4 to 10.5 or 10.6 ?

I need help !

I really want to upgrade my iMac with operation system 10.4 to higher level.

I was looking hard to find some answer in forums but I did not find anything helpful for me.

I own iPad 2 and iPod as well and I would like to have latest iTunes,more apps...

Info about my computer:

Model Name: Mac

Model Identifier: iMac5,2

Processor Name: Intel Core 2 Duo

Processor Speed: 1.83 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache (per processor): 2 MB

Memory: 1.25 GB

Bus Speed: 667 MHz

Boot ROM Version: IM52.0090.B09

SMC Version: 1.6f0

What is better option to buy- 10.5 (Leopard) or 10.6 (Snow Leopard) ?

Every answer is helpful!