It’s basically up to you, the iMac is in about the middle of the average product cycle for iMacs so it is possible (and rumored) that a new version will come out before the holiday season, there is supposedly a Mac event on October 14th coming up at which iMac may be introduced. If not then they may be introduced early next year.
Generally the form factor doesn’t change radically- all in one with a screen, thin. So you’ll basically be waiting for hardware upgrades such as more memory, better graphics card and increased hard-drive capacity (no guarantees). It’s up to you if increased performance is important to you, I would wait personally but I’m a ‘new and shiny whore’!
For future reference, you should check out the Mac Rumors Buyer’s Guide.