Avoid the G5s at this point. Even if they are dual processor ones, it's a platform that was end of life nearly 2 years ago now. Even if the machine has Apple Care, it's only going to be under warranty for another year at most, and then it will become very expensive to service if a problem occurs with the mainboard or processors. Also, most used ones would come with an older OS and iLife tools, meaning $129 added cost for Leopard and $79 for iLife 08.

Depending on what programs you want to run for video editing, a Mini might not be the best bet. Tools like Apple's Motion are GPU accelerated, and require a non chipset integrated video solution. Also, the Mini will be limited to one monitor, and you may want two screens worth of work area for editing. An iMac would be a good first step as others have suggested, and it can have a second monitor connected to it. Just be careful when looking at refurb units though, some iMacs also have integrated Intel graphics.