Since you have the eGo I would say the next step would be to get a higher voltage mod. I can recommend the Joker AV or the Silver Bullet. Others on here have the Buzz and like it a lot.
If you want to step up to a 5 volt manual, you will need to get a mod or get one of the 5V PT's you were talking about. The 510 atty that you use on your eGo will work on any unit that has a 510 connector.
You are limited to 3.7 volts with the eGo batt. No way that I know of to up the voltage of a battery. The 5Volt passthroughs will not work unless connected since there is no inline battery.
Those are called "passthroughs". They plug into your usb port on your computer or they can plug into your receptacles at home with an adaptor. They can also be used in your car using the cigarette lighter with an adaptor.
Cartomizers are made to be disposable. I am currently using an atty (Joye) that has been going strong for 6 months. So although the upfront cost is more for an atty I have found the longevity makes up for it.
Some people love attys and some believe cartos are given to us by god. Its a matter of preference.
The Vapor Talk store is currently waiting for a new shipment of e-liquids. All nic levels will be available when the new shipment arrives.
Edit: Christopher beat me to the punch. Man I'm slow today.