I agree, get as much life out of your current batteries. I started out with batteries that were supposedly dead and managed to squeeze out almost two more yaers out of them. I finally gave in and bought 8 new golf cart batteries which set me back on my "pay for themselves" goal, but I'm still on track for about 10-12 year payback.
It's funny, I actually calculated I can get them to pay for themselves quicker by adding additional panels since I've already payed for the battery/inverter/cable overhead. You'll probably be in that same category.
