Stacy, we're throwing a lot of specialized language at you, aren't we? Here are a few definitions, which might help you make your decisions.
510 - This is a particular model of atomizer. The threads will fit any other 510-type device, including the eGo. No other atomizer models will work with a 510 battery. There are many model numbers, but all 510 parts are generally interchangeable.
Atomizer - The part that does the work of vaporizing the eliquid. An atomizer is often used with a cartridge, or with a drip tip for direct dripping. The atomizer is the part that screws onto the battery.
Cartridge - A fiber-filled mouthpiece for the atomizer. You fill the cart with your choice of liquid. I always recommend blank carts and ordering your own liquid. This is much less expensive than prefilled carts!
Cartomizer - A cartridge and atomizer combination. A carto will hold quite a bit more liquid than a cart will, and is simple to use since the atomizer and cartridge are all one piece. The downside is that they don't last very long. However, they're also very inexpensive. Cartos are probably the most convenient way to vape your favorite liquid.
Direct Dripping - This is removing the cartridge altogether and dripping your liquid - 2-3 drops - directly onto the bridge in the atomizer. If you use a Drip Tip, you don't have to remove the mouthpiece every time you drip, just drip directly through the drip tip. Direct dripping is a common method for most experienced vapers. The reason is that you get more consistent vapor and flavor by controlling how much liquid is on your atty.
Cone - this is a part that threads down over the atomizer, onto the eGo battery. It's esthetic, and some say improves the draw and flavor of the device.
LR - Low Resistance - A special kind of atomizer that produces much more throat hit and vapor than a standard atomizer. These are used for 3.7V batteries to give the experience of using 5V batteries. They're a very good alternative for an eGo if you want to feel the vapor more in your lungs. In my experience LR attys don't last as long as standard ones, but they can be very satisfying to vape.
You asked for recommendations what to order and you've gotten excellent advice. I think what you want to end up with two eGo batteries, and at least five atomizers; you might also wish to try a few cartomizers, which are very convenient. Order blank attys and cartos, and refill them with your own liquid. Brian's suggestion to try a sample pack with different strengths is an excellent idea. You may not really be sure yet what your best strength is, but based on what you say, I doubt you will need to go any higher than 16/18mg.
It's hard to say how much liquid you need to last two months since that completely depends on how much you vape. You will vape more at lower strength than at higher strength. I think a good rule of thumb is 2-3 ml per day, so 120 to 180 ml of liquid for your two-month window. I started out using at least 3 ml per day, and now I probably use half that much. There is some waste filling carts and cartos; less waste direct dripping.
I hope some of this helps. I know how overwhelming trying to sort all this out can be at first! We've all spent a ton of money trying to find the combination that works best for each of us, and it's different for everyone. I really don't think you can go wrong buying an eGo. If they'd had them when I started vaping I would probably have had a much less expensive route to get where I am! By the way, 555 is my everyday vape too; I love the stuff.
Good luck in your ordering. Happy vaping!