The best vape is the one that keeps you off analogs, imho. If you are fully satisfied with your setup, there's little need to upgrade, except unless.. you just want to
ProVari is an exceptional device, and if you want it and are okay with the price, go for it - you have next to no chance of disliking it. I'm not sure that you need it, so I can't tell you that you should get it.
Atties and carts?! No.. you must have read something old or wrong. Don't go near that type of setup.
With a ProVari, you'll also need to get at least a couple of batteries that fit (either 18490 or 18650 with extension cap), a charger and pretty much every 510 attachment then fits on it. With a cheap adapter, if you really-really must, you can use the same atty-
tanks that go on your eGo-C.
But basically, you'd start with a few good standard resistance cartomizers, a couple of good quality standard resistance atomizers and go from there. There are various options - clearomizers of various sorts,
tanks for cartos, top feeder systems for atomizers - hell, you can try them all...
ETA:
http://www.youtube.com/watch?v=FG9DQbowrDA