ORIGINAL: Nitronutt
In a boat, on water and without obstructions (Like trees), you want speed and handling. Two 6-Cell packs just won't cut it. Go as big as you can in the beginning and you will be MUCH happier.
I highly recommend the IB3800 Cells. I have a super brain 977 and do charge my twin 8-cell IB3800 packs (Self made packs). The combo works great (977 and 2 8-cell IB3800 Packs). At some point, you may want an extra prop if you plan to launch/retrieve from very shallow banks (If you don't step in the water). The xtreme comes with a carbonfiber prop, so it is WAY better than stock. The plastic stock & carbon fiber prop are fairly easy to nick if going through mud with any shells or rocks, so buy one if you want one. They can get chewed up, but I have not ever shattered one.
I bought the Aquajet 775 last fall (After selling my Nitro Kyosho Jetstream 1000) and in the spring will have added all of the Xtreme upgrades anyway (If I don't go brushless). Steven, the owner over at Offshore Electrics frequents these forums and is a great resource on the boats in general. If you email him, he always responds.
P.S. The Superbrain 977 can operate from 120v house AND 12v auto, so you can charge at home or on the go (And has Lipo ability).
Fortunately, the lake we live on is completely sand bottom and no plants (it was man made. . . dub out for sand when they built an nearby overpass and thereafter built subdivion houses around it). Hopefully the prop will last a while but I may purchase an extra one just in case (are they easy to pull off and on?).
Picking the charger has been the only hold up to ordering the boat. I thought the super brain 977 may be extra helpful for me as a beginner since it has the LCD and tell me what is going on. About how long does it take to charge your 3800's with the 977?
What made you get the 775 originally? Were you unsatisfied with the speed of the 775 and thus decided to upgrade or did you just figure more speed is better.
Thanks for mentioning the 977 can operate from the house because that is exactly when my charging will be done. I had a few other make some recommendations only to find out that the chargers were DC only (in fairness to them, I didn't mention that AC was a requirement).