Can I dual boot Snow Leopard and Lion? I own a Mac Mini 3.1 with Snow Leopard installed. I would like to dual boot with Lion for development purposes.

I have partitioned my hardisk as described in Point (1) in this tutorial 

How to Install Dual Boot Mac OSX Lion and Snow Leopard

Can I now buy and download Lion from the apple store and then install it on the new partition?
You'll need to make your own bootable DVD/USB of the Lion installer to do so. This can be made from the AppStore download and is documented well by OSXDaily.

Lion USB Installer
Lion DVD Installer

Then hold ALT at system boot (before chime &amp; Apple logo) and select your install volume.