All Intel iMac's from 2006 thru Late 2011 are USB 2.0 and only the new 2012 when it hits the stores, will be equipped with USB 3.0.
For the various model spec's, see > Apple - Support - Technical Specifications
The free-standing (and freeware) database, Mactracker, may be easier to use:
than the Apple specs. MT is written more for the average user, yet still has full details. It also has the advantage of a consistent format. Apple's spec sheets have "evolved" their format over time, making head-to-head comparisons more challenging
MacTracker is available on the App Store but I've had a little trouble with that version. The version from the MT website works without issue.