Sorry guys, i am kinda newbie and could not find the answer in google.
I know there is 2G and 3G , are these related only to data transfer ? where does the voice goes ? if i set to 3g the voice will go in 3g frequency ?

http://lmgtfy.com/?q=difference+between+2G+and+3G

I will keep it simple - in GSM World:
2G: Voice Only or "Data Only". You can not surf the web while you are talking on the phone. -> Surf speed is slow

3G: Voice And Data concurrently. You "can" surf the web and you can talk on the phone at the same time. Surf Speed is "fast".
In some countries: You will be able to Video Call if your provider supports it.
(n900 does not supports video call "yet" except using gtalk or skype)

So: If you live in a place that have good 3g connection. Go with 3G.
If you live in a place that have a limited 3g converage and 2g. Go with 2G. (And turn on 3g only when necessary). Otherwise you will experience drop calls.

turning off 3G can help increase battery life a bit. for some light browsing 2G is good enough.

3G Mode uses WCDMA, and 3.5G HSPA (if available) instead of
2G also called GSM based communication (which is power efficient but much lower speed when compared to HSDPA) The data couterparts of 2G is GPRS and EDGE 2.5G. As mentioned in an earlier post,
in 2G mode does not usually support data access through cellular network when a cellular voice call is active. Ofcourse one can use WiFi if available.