Well, I've recently bought a second-hand iMac (3.4Ghz i7 16GB ram 27"). Like you, I pontificated for a week or so. I trolled these forums, looked for any clues about a new launch. Then I realized the new launch may only be June, some people told me even October. Then I realized that even if a new iMac launched today, South Africa would only get it like, a month or two later. And I'm getting a little weary of my old Core 2 Duo Macbook's speed, so, if you can afford it and need it now for something, get it now. But seriously ask the question - do you really NEED a new machine? Can you live with your current setup?
But - if you wait, you could end up waiting a good long while. Is it worth it? From what I gather (my machine is probably getting to me tomorrow) the current gen are pretty rad. I'd suggest though, getting an SSD instead of HDD. That seems to be the best kind of upgrade at this point.