Okey doke.
Those 2200mah Elites that you've linked to don't really have the capacity for use in a truggy, but they do some larger cells which would be ideal. I'd completely forgotten about the Elite LiPos...they're meant to offer very decent performance for the money. So...
...here's a quick "LiPo 101" for ya! They're really not that tricky.
LiPo stands for "Lithium Polymer" as you'll already know, and they have a considerably higher energy density than NiMH; this means that they can pack more capacity into a smaller and lighter package. Typicaly, a LiPo cell will weigh roughly 33% less than a NiMH of the same capacity, while giving greater runtime because they're much more efficient when discharging. They also have a much flatter discharge curve, meaning that they're output near-enough their maximum voltage right until they're almost depleted...unlike NiMH cells, which tend to run down slowly once they've been run a few times. And the right LiPo will be able to provide higher voltage under load than a NiMH, because of their much lower internal resistance.
The thing that usually confuses some people is the "C" and "S" ratings. "S", as you know, simply means "series" and refers to the number of cells in series. So a "3S" pack of LiPo cells will total 11.1V nominal (3 x 3.7V).
The "C" rating is very much a critial specification of any LiPo pack, and determines whether it's the right one to buy. When you see a pack that's rated for 20C for example, that means that the pack is capable of discharging at twenty times the pack's capacity. So a 2000mah, 20C pack would be capable of 40A discharge (20 x 2). A 4500mah 30C pack would be able to do 135A (30 x 4.5). You'll see that most packs have a "constant" rating, and a "burst" rating; it's best to specify a pack that will barely exceed its constant rating in a burst, if you can. Most motors have a maximum rated current draw, so you can choose your LiPos based on that...though there are always going to be better LiPos than others.
If you want utterly insane power (and about 50mph, possibly a tad more...I'll put the numbers through the BrianG calc later)), then a pair of the Elite 3S 4800mah 25C constant packs (scroll down a bit) would be ideal:
http://www.cheapbatterypacks.com/main.asp?sid=1088352&pgid=showlipos&man=ELT&cat=E25.
You'd wire them in series for 6S, and should get pretty big runtimes with the Castle motor...probably around 25-30 mins without a drop in power until right at the end of the run. Or you could go for a pair of 2S packs of the same cells for 4S. It'd give you slightly more sensible speeds and be cheaper (around 35mph)...but do you want to be sensible lol? The configuration you suggested would be good for these packs, as they wouldn't have the weight penalty of NiMH over the back of the chassis.
Hopefully that's of some sort of help...just say if it doesn't make any sense lol. By the way, have you bought that Ansmann charger yet? If you have, you'll need to get a hold of a LiPo balancer.
