I'm guessing its an 8 speed out back?

No matter what cassette you look at, you have to factor in the chain rings up front. A safe guess (since its a MTB) is that you have maybe a 42, 34, 24 up front. That being said, and that you want to make it more street friendly for flatter terrain, I would run an 12/23 cassette.

Overall, that gearing would give you lots of gears but the ratios would be relatively close for flat ground riding. I did this on my Giant Innova. The original gearing was 42/34/24 x 11/28 giving me a huge gear range. I found that I never used he very low and very high gears so I switched the cassette to a 12/21. The gear ratios were much closer and the bike is really fun to ride now.

Hope this helps. Let us know what you decide and how it works for you.