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1199$ 21.5" iMac
Intel Core i3-2100 (3.1GHz)
AMD 6490M with 256MB GDDR5
750GB HD
2x2GB RAM; option for 4x2GB
1499$ 21.5" iMac
Intel Core i5-2400S (2.5/3.3GHz); option for Core i5-2500S (2.7/3.7GHz)
AMD 6750M with 512MB GDDR5
1.5TB HD; option for 3TB
2x2GB RAM: option for 4x2GB
1699$ 27" iMac
Intel Core i5-2400 (3.1/3.4GHz)
AMD 6750M with 512MB GDDR5; option for AMD 6950M with 1024MB GDDR5
1.5TB HD; option for 3TB
2x2GB RAM; options for 4x2GB, 2x4GB and 4x4GB
1999$ 27" iMac
Intel Core i7-2600 (3.4/3.8GHz)
AMD 6950M with 1024MB GDDR5
1.5TB HD; option for 3TB
2x4GB RAM; option for 4x4GB
Okay, some explanations. Apple could do like they did with 2010 iMac and offer 5670 (aka 5730M) in base iMac. It's a bit faster than 6550M (no benches for 6570M yet) but nothing super special. 65xxM only come with GDDR5 or DDR3 so GDDR5 is the only viable choice for Apple or it will be a downgrade.
Why quad core in 21.5"? Because there is only one dual core i5 and it's 35W T-series chip. All other i5s are quads. In terms of price and TDP, quad core would suit there perfectly. S-series has TDP of 65W which is less than current Clardales with TDP of 73W. The price for i5-2500S is 216$, 78$
less than the current BTO option i5-680.
I didn't want to add any SSD things there because IMO they are too speculative. I'm sure Apple will offer some kind of an SSD like they do now but what will it be is a mystery. Maybe all iMacs come with 64GB or 128GB SSD for OS X, who knows? Everyone can make their own guesses about this.
Why no 3TB option? Remember how long it took to get 2TB as BTO? Currently there is only one 3TB 7200rpm drive AFAIK. It's Hitachi and what I've seen, its availability is bad, just like all 3TB drives. 2TB drives still cost too much to be in stock models.
Next, RAM. I know Sandy Bridge can take up to 32GB of RAM. However, 8GB SODIMMs are more or less non-existent. Even if they existed, they would cost more than the iMac itself. Adding 32GB as DIY will likely be possible unless Apple limits it with firmware but it will take awhile before you can get affordable 8GB SODIMMs.
4GB modules are starting to be very affordable right now. I would expect 8GB in high-end iMac as the price is about the same what 2x2GB cost a year ago for example. Why not in other models? Usually Apple wants models to have differences. 21.5" is pretty much out of question as Apple wants 27" to be better. Low-end 27" might already be a tight profit for Apple due to the expensive screen so adding more RAM doesn't sound very Apple-ish. However, I wouldn't be surprised if we didn't see 8GB as standard in any iMac.
What about CPUs? There are only two i3s so those are easy to predict. i5s are too expensive (especially S models which are suitable for 21.5") for other than the high-end. It is possible that the 27" goes quad core only but even the cheapest i5 (2300, 2.8/3.1GHz) is 39$ more than the current i3-550 used in low-end 27". Like I said before, low-end 27" may already be low-profit machine for Apple so every $ counts. It would also make the difference between low-end and high-end 27" very small.
As for high-end iMac's CPUs, Apple could use i5-2400 (3.1/3.4GHz) or i5-2300 instead of i5-2500 but the price of 2500 (205$) is the closest to the current i5-760 (205$

).
Some of my and others' thoughts about the GPUs can be found
here! We still don't know the TDPs which makes predicting a lot harder.
The last but not least. What about Blu-Ray, Light Peak, USB 3.0, iMac Touch and new form factor?
According to
this, Steve does not like Blu-Ray due to several restrictions. I'm going to say we won't see Blu-Ray, at least not in the next update.
In IDF 2010, Intel said computers with Light Peak will come in 2012. We are living 2011 now so yeah, no Light Peak, yet. The rumor about Light Peak coming in early 2011 had no proofs supporting it. Only a month or so before, Intel clearly stated that it will be late 2011 for the first parts and 2012 for computers with LP.
Sandy Bridge does not support USB 3.0 natively so don't get your hopes up. Steve said USB 3.0 isn't taking off, which doesn't sound too good when it comes to Macs with USB 3.0. It is possible that he lied which isn't a surprise. Remember when he said there won't be App Store for Mac? Well, there is now. I still wouldn't bet on USB 3.0 though, I doubt we will see it.
I'm going to put this straight, touch does not work well with a big screen. Keeping your arms up all the time will be painful and that is not what using a computer should be. I've see the patents of incline iMac but think about the glossy screen. Now you can work okay with ceiling light on but with incline iMac, the light would cause some serious reflections. Let alone what your neck and back would say when leaning forward all the time! Then there are things like finger prints, OS and app support which makes touch even more useless.
A new form factor doesn't sound plausible to me. iMacs got new form factors in Oct 2009, why would Apple change it now when it has been so little time? Making 21.5" an inch bigger isn't going to help anything, especially with the same 1920x1080 resolution. I know some of you drool after the 16:10 ratio but I doubt we are going to see that. 24" iMac was even more expensive than the current 27!
I'd like to add that this is just my speculation. I have no insider information about future Apple products. Everyone is free to do their own speculation and of course debate about some point I made.
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EDIT: Since MBPs got Thunderbolt (aka Light Peak), it is very likely that iMacs will get it as well. It should replace the Mini DisplayPort.
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EDIT2: Updated the CPUs and GPUs due to the MBP update. So many MBPs went quad core and got nice CPUs in general so iMacs must follow them in order to fight against. I put that high-end will have i7-2600 although I have some doubts about that since it costs as much as the current i7 and other parts cost about the same as well. It's possible that Apple will use i5-2500 instead and offer the i7 as a BTO.