When I buy a new plant, I add it to my list here in the database, I have set up different categories (see my profile) I add notes like where and when purchased, you can also add where planted and any other notes.
For detailing my garden progress, I use the blog feature here. I currently have it start back 16 pages when I first moved to this property almost 6 years ago. Great way to add progress notes and photos, lots of before and after shots too. I love going back to the beginning and see how far it's come!