What tires and rims came with the bike?

When you say you want to go faster, do you mean on the street? If you want to increase speed on the street, you'll loose off-road traction.

Something like this would work well on the street but off-road capability would be very limited. Depending on the rim you have, you might have to get the 700x37 size. Look on your rim for a marking on your rim that says something like "622 x some_number". Then we can have an idea of what tire size you can use.
http://www.conti-online.com/generato...ontact_en.html