what is the latest OS can be installed on Mac mini Intel

I have Mac Mini with the below specs. It is running on 10.4.11 Tiger now. I wish to know to what is the maximum OS can I upgrade to? And where can I download that OS (official copy only. paid or free). I found Snow Leopard available in Apple Store, but i can't download it. I don't want it to be shipped, as it will take time.

Hardware Overview:

Model Name: Mac mini
Model Identifier: Macmini1,1
Processor Name: Intel Core Duo
Processor Speed: 1.83 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache (per processor): 2 MB
Memory: 1 GB
Bus Speed: 667 MHz
Boot ROM Version: MM11.0055.B08
SMC Version: 1.3f4
Serial Number: YM632023WKN