According to Wikipedia, DOCSIS 3.0 is, concerning speed, essentially DOCSIS 2.0 without a maximum number of channels. So depending on how many channels your modem has, it is that many times faster than DOCSIS 2.0.
Wikipedia states that there's usually either 4 or 8 channels which would throughput approx. 175 or 359 Mbps, which is about 21 MBps or 42 MBps, respectively.
Which is a LOT faster.
Practically though, you might not notice the difference unless you stream 1080p Youtube or test out Linux distros on a regular basis. But even more practically, a DOCSIS 3.0 modem's about $100 or €60-ish. That's not much unless you're strapped for cash like me. Even if it turns out to be unnoticably faster (which it may in extremely urban or rural areas) then you basically don't get to buy 5 DVDs.
Which isn't a problem since you can rent movies for iTunes on that superfast network.