I wudnt buy a fake battery, they dont perform as well as a genuine 1 wud. And considering this is an n900 it requires a lot of juice for the stuff it does or that u will do it on it.
Also on another note, i wud highly advise folk against using a cheap/fake car charger for the n900 as it causes interference in the audiio quality whilst using fm transmitter an it also never charges it fully. Im gona get a genuine 1.
(i never bought this cheap 1, it was given to me as sme1 had a few)