Well I've just got a higher capacity unbranded battery from
www.keene.co.uk and it cost about £35. Now the best price for a JVC high capacity battery (1600Ma which is double that of the supplied battery) costs about£70 - £100.
So I reckon it's still better to get 2 unbranded batteries which would outperform considerably the length of 1 JVC original battery.
However, from previous experience you shouldn't leave these batteries unused for months on end as they do deteriorate and then die. This happens with branded and unbranded batts but possibly less with branded ones.
Good luck with the camcorder, I think you'll be pleasenty surprised with the quality. Even in lower lighted rooms I get reasonable results but most DV cams seem less sensitive to light compared to older Video 8, SVHS or VHS-C camcorders.
So next on you list.... Powerfull pc with firewire socket, second large hard drive and a DVD burner!