For the 800 they recommend 1000mA. I would put it like this. You can use them, but it is recommended to use the original charger as main charger. I think my BB chargers are only 750mA but I also still use them for the Lumia if they are closer... Bear in mind that a USB port in a PC only gives out around 300mA.
It will take a little longer to charge and My experience is that the charged battery lasts longer when charged with a 1000mA (Or my 2000mA iPad charger)
So, it can be used. But it is not the best option.
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