Max charging current on N900 is limited to 800 (or 900, can't remember exactly and too lazy to check the kernel code) mA when a wallcharger is connected, so, no matter the maximum current that charger can provide, the battery will not charge any faster. Also, there is a "charging curve" - the charging current depends on the current charge.

In short - there in no gain in getting a charger that can provide higher output current than ac10, besides that it won't get that hot