The 10.6 Snow Leopard DVD is in the Apple online store. You can get it for $19.99. You will need to be running 10.6.8 to access the App Store in order to download Lion or Mountain Lion. http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
System Requirements for 10.6:
- Mac computer with an Intel processor
- 1GB of memory
- 5GB of available disk space
- DVD drive for installation
- Some features require a compatible internet service provider: fees may apply.
You can upgrade from 10.4 straight to 10.6 with no problems as long as you have at least 1gb of RAM. Any program that runs under 10.4 should run under10.6. See this list for third party programs compatibility with 10.6: http://snowleopard.wikidot.com/
You might have to upgrade some drivers for printers, etc.... And you will have to install Rosetta if you have any Power PC applications
I recently upgraded a MacBook (Early 2006 model 1,1) straight from 10.4 to 10.6.
A retail 10.5 Leopard disk will run you over $200