I'm not very good on assemble batteries as I never done it and I don't want to make mistakes an throw them away!
Thanks for the hint but I think I'll look for assembled pack: I'm not racing so I don't really need super extra performance, I just need enough to have much fun!
I saw the ORION Rocket Stick Pack 7,2V-4500mAh and they seem to be good enough (specially compared to what I have now...) only need to replace the connector with Deans.
The ZX-5 SP seems to be a real nice car as I saw around: I ordered it online from a UK shop as it's not available in Italy yet, and waiting for it to get here!