If you don't have batteries now I would truly consider the A123's. I fly with Altavillan and he's using them in and on everything. His 35% planes are taking 100 to 150 millamps after a hard 3D flight and that just impresses the heck out of me. For those that are going to fly a 2.4 system the A123 is a far better battery choice since it maintains a higher voltage for much longer than anything else out there. Those 2.4 systems will dump the plane in a heartbeat if the battery voltage drops below a certain threshold. That's a terribe reason to lose a plane. If you're flying 72 mhz there's no real benefit (imo) to using A123 batteries. Altavillan and I had just this discussion yesterday.

If you want to go with nimh then an 800 mAh ignition battery on most single cylinder engines will be great for about 2.5 to 3 continuous hours of flying. You could go with a nicad instead for the ignition and be able to fast field charge it if needed. Same capacity, same amount of running time. I switched from Li-ons to nicads at my ignitions for that reason. Cheaper too. I would not use less than a 2,400 mAh receiver battery. As always, monitor you battery's condition and level to know where you stand.