My original plan is to order the maxed 2012 27" iMac, but then I realized that in less than 4 month from the iMac is shipped, next gen Intel CPU will be out. It's reported the Haswell will have 10-20% CPU performance improvement (under the same frequency), and even more for multithreading. I am now wondering if I should wait for the new CPU architecture......Any advise?
I think it would be foolish for you to wait in light of future CPU updates:
1- Apple won't likely be releasing updated iMac before next fall.
2- The speed increase you would get is negligible.
3- Now that the new design direction and Thunderbolt are implemented, there are few things we can reasonably predict as "the next big thing" for iMacs. The only one that comes to my mind is the Retina display, which by no means is likely to see the light any time soon, and it's certainly not so much needed in a desktop system like this.
Net, next generation of iMacs will very likely be "more of the same": a bit thinner design, a bit more power, a bit more fusion space, etc. Which is to say, like every other technological thing, something a bit more advanced for the same price. But no predictable breakthroughs: we already had one with this release, and that is the thin design (like/need it or not that's another story).
I don't see why you should be waiting: that is, if you need an iMac. If you don't, just don't buy it. But don't get into the waiting game one month after a product was released.
If it was June of next year i would give you the same advice.