Most important thing is to learn how to collimate the scope. You will find clear instructions online, e.g., in the Orion XT Dob manuals, or in a Sky & Telescope article on the topic: "collimating Newtonian". I'd recommend a visual Cheshire style collimating tool over fancier gadgets like lasers because it's more reliable; a laser collimator will itself need collimating. (Without regular collimation a Newtonian will perform poorly. Collimation for a telescope is kind of like wheel alignment for your car.)
There is lots to see with an 8" f/6 Dob, which is a fantastic insturment. It's way too early to think about going bigger. The 2x barlow is a good addition but you'll be able to make better use of it if you also get a 12-15mm eyepiece. The 10mm Plossl barlowed will push the magnification too high on most nights. Also plossls around 10mm are at the limit of what I consider comfortable because of the short eye relief / small eye lens. No need for expensive gear: a 15mm fully multi-coated plossl for <~ $50 would do nicely.
Seeing you're in Brisbane, it'd be worth visiting Astro Pete's telescope shop, see
http://astropetes.com.au/ and having a chat with the guy. By all accounts he's an exceptionally helpful, honest and knowledgeable astro-gear dealer. I'm sure he'd give you good advice.
What make barlow did you get btw?