Assuming you're not falling-back to a non-3G connection, you are probably connecting to "plain 3G" i.e. UMTS W-CDMA. The N900 is promised to support HSPA (high speed packet access). The download part of this, HSDPA, is sometimes called 3.5G by marketing people.

I have used my N800 (with a cellphone via bluetooth) on both 3G and HSPA networks, and the difference is unbelievable. HSPA browsing feels almost like WiFi, whereas plain 3G sometimes can be very clunky.

Today's HSDPA phones support maximum download speeds of 3.6 or 7.2 MBit/s but it doesn't matter much because (depending on a number of factors) you don't realistically achieve the maximum. A throughput of 2 Mbit/s is just as good whether your phone is capable of 3.6 or 7.2 MBit/s.

Roger