All great advice. How are you going to charge your new battery? I asume (hate that word) you have the original wall charger. Do you belong to a club, or have any local flyers near? Lots of 600ma 4.8v packs around, which is all you need, and can charge from your wall charger. Someone local may want to up size, and give you a recent 600ma.
If you go to a 5cell pack at this time, look at how you are going to properly charge it. A 5 cell pack, as earlier stated will be benificial in the future although not necissary now. But don't think your wall charger will charge it properly! That could become a dangerous, false sense of security. In time a reasonably good charger/cycler will show, and prove it's worth. But first go injoy your LT40 they are great. 600mah, to 1100mah at 4.8V has my vote. On 72mhz equipment.
-Snuts-