I use mainly NiMH batts for RX packs. I get 1000 -1200mAh packs and those will typically last me 5 or more flights (~ 10 mins or so each) in my planes, which have 3-4 Futaba S3004 servos on them (standard BB servos). I don't do 3D or much aerobatic stuff yet, and flying "harder" will run that batts down quicker. I put a 1200mAh NiMH pack in my latest SPAD, and got 9-10 flights (averaging about 8-9 mins each in the air) before my VoltWatch batt meter read in the "low" range. I have a 2200mAh NiMH pack in my Raptor 50 Heli, but that has 5 high torque digital servos and a gyro, so I get 4-5 flights out of that.

I'd say if you're just starting out, and already have a 600mAh NiCd pack, try it and see how many flights you typically get out of it, and upgrade it if you want more. If you're looking to get a pack for your first plane, I'd recommend a 1000-1200mAh NiMH pack. I believe that is a pretty good tradeoff between additional weight and additional capacity. I find I can usually fly until I'm ready to take a break, and I can charge my batts while I BS with the guys at the club!

I'd highly recommend getting a good peak charger for use at home and the field. You will find youself needing to recharge your RX or TX batt at the field sooner or later. You can just hook this charger to your car battery, or most guys get a field box with a 12V sealed lead acid battery inside to power an electric starter, fuel pump, and the charger. Most guys get something like a 7Ah 12V batt for their box, but I've seen 4Ah batts, and I have a 12Ah batt in mine.

Here's a couple of good peak chargers... I have the dual output (two batts at once) MRC SuperBrain 969 Pro, but the single output 959 is half the price and also comes with an AC supply for home use. The Hobbico is a good value on a dual output peak charger, but it has no AC supply, just a 12V DC input...

http://www2.towerhobbies.com/cgi-bin...&I=LXEVC7&P=ML
http://www2.towerhobbies.com/cgi-bin...&I=LXAVN1&P=ML
http://www2.towerhobbies.com/cgi-bin...&I=LXCTZ5&P=ML

I also highly recommend installing one of the VoltWatch batt monitors on you plane(s). I have one on all my RC aircraft, and it makes monitoring/checking your RX batt pack a no brainer... just move both TX sticks around to get all your servos moving and watch the meter to see how low it goes under the load... if you're getting in the "low" range with all the servos going, it's time to charge up before you fly again...

http://www2.towerhobbies.com/cgi-bin...&I=LXMZ56&P=ML
http://www2.towerhobbies.com/cgi-bin...&I=LXHDJ2&P=ML

Hope that helps...