04-03-2011, 10:09 PM
Apple's infamous for giving false information about their products that are limited to their actual capacities. Older iMacs were listed as 4gb ram max, when you could put 6gb in them. When the lynnfield iMacs first came out, apple only listed them as being able to take up to 1066mhz ram, while you could put 1333mhz in them. The 5750 and 5670 gpus in the iMacs are actually mobility 5730 and mobility 5850, etc.
32.0GB (8GB x 4) PC3-10600 DDR3 1333MHz S... (1333DDR3S32S) at OWC
Theoretically the 21.5inch form factor doesn't matter for heat, as you could crank up the fans to higher rpms (say 2k or 3krpm for each the cpu or gpu) and support the added heat, but apple loves to keep their fans at very low rpm to keep their grippy consumers happy about audible sound.
However the size has everything to do with it. Apple knew it'd be hard to get high end quad cores in an iMac unless they made it larger. Looking at the sheer size of the heatsinks apple used when given the larger chassis size is clear evidence of it.
Anyways since sandybridge will introduce 65tdp quad core desktop cpus like the current 65tdp clarkdals in the 21.5 inch iMacs, there should be no reason not to have them in the next iMac revision.
Of course I know that the stated maximum support is likely different... but that's not the point either.
Why doesn't it matter? You were wondering why 8gb was the displayed ram limit for 21.5 inch iMacs-- unless you like buying severely overpriced ram "options" the apple store offers you.