ORIGINAL: MountainMotor
If you want fast run it on 4S, if you want insane run it on 6S. I have 2X3S Zippy's, similar to what traxxas4ever posted up that I run in my Flux and it has all the ''punch'' you can handle. I run my Revo on a single 6S and it is rediculous. I would say go with Turnigy or Zippy's and go with either 2X2S or 2X3S, depending on how fast you really want. Like the other guys said, Hobbyking has some great deals on them.
[youtube]http://www.youtube.com/watch?v=1egTGCtZgxE[/youtube]
That was done with the 2200kv castle combo in my truggy (sorry the vid quality sucks I used a digital camera held in hand with my radio which was also the cause of the crack up at the end)
That was done on 4s(the ones I linked above) and honestly the wheelies were at like 30% throttle. At the gearing I never actually hit full speed seeing at around 50% throttle if I punched it to 100% it would wheelie and flip, and if I incremented it the tires would baloon up and at around 70% the front end would take off the ground and it would flip. The gearing in it at the time was the same gearing in a jammin x1 CR buggy. I've replaces the spur since with one that I think 20+ teeth more, and the wheels still baloon like mad but now the power is useable
I can't imagine this system at 6s in all truth
EDIT: Swore I hit the you tube link instead of quote oops