You could get one of the best there is, which is what I have. A Chuck from Thagbuilt.com. The Chucks come in 3.7v or 6v, and the 3.7v is an ACTUAL true 3.7v mod, unlike the models I listed below which run at 3.0-3.2 volts instead of 3.7. Also the Chuck is customizable. You can put any graphic you want on it and can pick the color of the body.
Also, the 3.7v chuck (little chuck) and the big chuck (6v or 3.7v) both use outside batteries, meaning that if you need a new battery you can just go to radioshack to get them instead of having to buy a whole new e-cig battery. Much more convenient.
You should really try out a 510 to see if you like it. 510 is one of the most popular atty setups. The 510 I pretty much the most available model out there. Most all vendor sites you go to will carry the parts.
With that said, the Riva is a great choice. Or the eGo/Tornado. I would suggest a mega battery for improved (GREATLY IMPROVED) battery life. All of those models are 510 compatible, and if you use the LOW RESISTENCE 510 attys on them, it really doesn't get much better than that. Just my suggestion, good luck!