Joye 510 - you are already where many people here start. 510 is probably the most supported model parts in the reasonably priced retail price marketplace.
510 and KR808D-1 are the two most popular and most recommended e-cigs here for people who want a slim cylindrical ecig. Anything smaller or weaker or mystery model "name" brands" are not recommended (for example ecigs that are not sold by multiple vendors at reasonable prices, ecig vendors that do not offer blank carts or cartos, any that charge more than $20 for a battery - we do not recommend those).
Joye 510 is slightly slimmer, comes as a 3-piece but there are lots of blank cartomizers available for making it a 2-piece (how I use it), has rather low battery life with its standard batteries but you can also get 510 mega xl batteries which give it 50-60% more battery charge life. Manual batteries (with the button) are almost always recommended. More sealed against death by juice leaks, simpler mechanism, less work, won't turn on by accident from wind or vibration
KR808D-1 is a bit fatter and is a dedicated 2-piece. Many prefilled cartomizers available, smaller selection of blank cartos. Standard battery lasts as long as 510 mega xl batteries. Manual batteries recommended. Also sold by Vapor4Life.com in colors and they sell a lot of prefilled cartos.
Prefilled cartos are just a small convenience since unless you are made of money you will be adding ejuice to make them last. if you don't add juice a carto might only last part of a day before it gets too dry and you singe it. If you add juice and keep it damp a cartomizer can last days or longer.
Comparison
http://www.e-cigarette-forum.com/fo...st-electronic-cigarette-first-time-vaper.html
If you want to improve what you have, try blank cartomizers. They take a bit more time to fill the first time and are a repeat expense but you won't have to add juice as often. A cartomizer is a disposable 1 piece cart with atomizer built inside - if you keep them damp with juice you can get days or longer out of one. I have gotten more than 2 weeks out of one. Usually more like 4-7 days. Cartomizers cost less than $2 apiece. I like the convenience of them being vapable longer before juice needs to be added. You want soft-cap cartomizers like these so you can pop the soft cap out (stick an unbent paper in the mouth cap hole just enough to pop it out so you don't catch the stuffing or anything else).
http://www.litecigusa.net/510_Blank_Cartomizers_p/510bogesoftcartosb.htm
And consider using the 510 mega xl batteries (makes it about 138mm long with atomizer and cart or about 128mm long with a standard Boge soft-cap cartomizer). They use a different mega PCC (portable charging case) for charging when you are away from USB/AC.
If you want to step up to something stronger with more battery life, consider the fat battery version of 510 like Ego, Ego-T, Riva 510. These fat-battery models can also use harder-hitting hotter Low Resistance LR parts if you have any interest in those.
