Wow.I like that lineup. I if they do any thing like this I am going to buy another one. Sadly I don't think that a Quad i5 is going in any 21.5 mac.
I could see the high end i3 540 or low end dual core i5 650 as a upgrade in the 21.5's base. The dual i5 650 as the starting processor and the higher end 661 as the processor upgrade for the 1500 dollar 21.5. I doubt that the base will get a 1TB same as the 1500 imac, maybe 640GB or 750GB. I hope I am wrong though, and they do something along the lines that you posted.
The biggest bang for your buck is the i3 540. It looks like the base imac will be the biggest winner for the next refresh. Regardless.
i3 - 540 - 3.06 GHz - $133
i5 - 650 - 3.20GHz - $176
i5 - 660 - 3.33Ghz - $196
and
i5 - 750 - 2.66Ghz - $196
looking at the price, other than i3 - 540, other i5 dual cores are near to i5 750.
it is fair say that i5 - 750 is good choice for 21.5" $1499 model.
Only constraint is quad core i5 heat and thermal problems in 21.5"