Check out the TrueRc. The 8000mah 3s and 2s packs on this page are great for the money. Ive beeen using them for quite a while and for the price they are unbeatable IMO.
http://home.comcast.net/~truerc/read...eady_packs.htm
As for a charger. I like the Super Brain 989 for value to performance. The 977 you have will work, but it will only charge the 2s pack at 3 amps and the 3s packs a 2.5 amps. that means about 2.5 - 3 hours to charge a pack, but you can charge 2 at a time.
If you do go to 6s on the 9920 and the 9xl make sure you drop your gearing a few teeth to keep the temps in check. I really think you will notice a huge difference just by going to 5s from the NiMH you have been running.