What would be good for a 9920/10xl in an gmaxx.

5S lipo.

what about 2 3s packs 3700 mah?

I would personally want more mah.

Either will work fine. If you get 6s, you'd want to gear down some. It would give better power and efficiency and runtime than 5s as long as it is geared right.

When you say 6s, do you mean two 3s packs? Showing my stupidity with lipo! sorry!

6s = 6 cells in series. You can use two 3s in series, one 2s and one 4s in series, etc as long as they have the same capacity (Ah). More voltage will also pull more current (although you tend not to punch it as hard, so that is easier on batts), so make sure your batts have a high enough C rating so they won't be overloaded.

Ok. What is "c" rating? Does Mike sell 3s'? In time i would like to get two 3s packs that would equal about 16 3800 cells. Would this be possible? Also, when going Lipo, what all do i need to maintain the lipos? I think my MRC 977 would charge them, but do I need a discharger, balancer, what else? thanks!

Mike should be able to get you just about any battery configuration you are looking for. If you do plan on running 6s just make sure you get two 3s packs because the 977 charger only charges up to a 3s if I'm not mistaken.
I would also recomend getting a balancer and an LVC.

Thanks, LVC? And what would be a good balancer?

This is the LVC that most people seem to use including myself. http://66.241.195.91/lithium/lithium.asp?path1=lithium
And this is the balancer that I use. http://www.maxamps.com/index.html?lmd=38888.421285 (click on chargers and misc)

thanks man! great help! So the first link is for the li saver that connects to your rx and esc, right? Also, if i run two packs in series, how many li savers will i need? one or two? I'm assuming one? Also, the balancer is it the first one? item# LBA6? Sorry for all the questions. Lot to learn here! will i need a high voltage cut off?

Yes, it goes inline. So the esc plugs into it and then it plugs into the RX where the esc would.

So I can wire two MaxAmps Lipoly 4000mah 3S 11.1V packs together in series and be ok?

Sure.