if your phone shows a 3G or 3.5G connection, you are using the 3G network, built at huge expense by the operators (for both licence and infrastructure).

2.5G is a way of wringing a bit more performance from the old 2G network.

Of course, just cos your operator advertises their 3G network as 'upto 7.2Mb/s' doesn't mean you will get anywhere near the theoretical max in practice. Use www.speedtest.net over a few tests in different locations to see what speeds you can achieve in the real world - but you can trust the hone when it shows you the connection type, it is telling you the type of physical data connection it has established.