This is likely a coverage issue. If you are in an area with HSPA+ coverage built up, you should be able to take advantage of the higher speeds without any intervention from you.

In my experience, the APN you select (most folks seem to select between epc and internet2) does not really make a huge difference in speed (though ymmv)

The fastest speed I have ever seen on my n900 utilizing the T-mobile USA network was about 3Mbps down. Like I said, the speeds you get are highly variable depending on the coverage in your area. Finally 3Mbps is slower than hyped, but when I get those speeds, it is certainly fast enough for me to forgo paying for wireless when I am on the road...