Well, yes, indeed, there's a lot of confusion because there are a number of different types of E-cigs out there. Perhaps the most important thing you need to know is that, except for a few cases, the number (510, 808, 901, etc.) indicates a different type of connector, thus almost anything with a number that is different than yours won't work, at least not without an adapter.
- The 306 and 510 are the same threads. However, 306 atomizers are made for those "penstyle" E-cigs you mentioned and have an exposed bridge (the metal mesh that dips into the cartridge to soak up liquid), while the 510 atomizers are recessed up into the body of the atomizer. This is because penstyle E-cigs have a cartridge that fits over top of the 306 atomizer, while the 510 carts are designed to fit inside the 510 atomizer tube. So you can't interchange the cartridges, but you can get 306 drip tips to fit over top of the 306 atomizers to use on a 510 or eGo. The differences aren't really that important beyond know what you've got and what will fit on it, and if you have an eGo or 510, you can use all 510 accessories on the eGo, and most eGo accessories on the 510 though it's not recommended and looks weird anyway. It's recommended you only use straight 510 attachments on a 510 battery. However you will have no problems using any 510 attachments on an eGo.
WOW THANKS! I'm hardly a Noob but that 306/510 explanation is much appreciated! I always wondered about those little buggers!