I just wanted to add my own experience.
I wanted to do this too, upgrade from the 1TB to the 2TB drive in an iMac EMC 2374. A very popular and a bit older 27" iMac.
Other than this post, I found it near impossible to determine the correct hard drive to use for success.
My replacement went like clock work. More smoothly than I ever imagined.
I use the Seagate Barracuda STBD2000101 drive, available from my local Best Buy. This drive has the 4 pin thermal connector. An 2TB drive that works flawlessly in the 27" iMac EMC 2374.
PARTS?
All I did was go over to my local Home Depot and buy 2 "Brutus Suction Cups" Model # 75000-6, Store SKU # 253240 for $6.98 each. They are heavy duty 4.5" rubber suction cups.
I also bought at Home Depot the "Husky 8-IN-1 Torx Screwdriver Set" Model # 74502, Store SKU # 165355 for $5.97.
This has the 2 Torx drivers you need, in this set.
I followed the various online examples to remove the iMac screen to get to the hard drive. Mainly, I followed the instructions from the below linked web site to dismantle the iMac.
http://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2309+and+2374+Hard+Drive+Replacement/1634/1#s8583
NOTE:
IMPORTANT:
To make the switch, I wanted to "FIRST" make sure I had a current TIME MACHINE backup, to restore from after the hard drive switch. So you should make sure you do a Time Machine backup before pulling the power plug and swapping out the hard drives.
Naturally, TIME MACHINE backup to an ""EXTERNAL HARD DRIVE"".
I used the "Iomega 2TB external firewire hard drive - mac version" for the time machine backup.
(My normally used time machine drive.)
I then followed the guide from the website (linked above) to replace the 1TB with my new Best Buy Seagate Barracuda STBD2000101 2TB hard drive.
I put the iMac back together.
Grabbed my iMac Install DVD, turned on the iMac and inserted the DVD.
Then I had to manually power-OFF the iMac and power-ON while holding down the "C" key on the keyboard, so that the iMAc would boot from the "mac os x install DVD".
(otherwise the system comes up eventually with only a flashing FOLDER icon, if you try to power on the iMac not using the original INSTALL DVD.
So booting holding down the "C" key with the install DVD, the system finally comes up with the install screen after some time waiting.
You can hear the DVD clicking and booting as it loads up.
Then, when the DVD loads you will have to select your language and continue.
STOP!!!!
DO NOT install the mac OS from the boot DVD.
If you look to the upper left of your screen you see the menu selections.
Select utilities and DISK UTILITIES to PARTITION your new 2TB hard drive.
On mine, I just made one big 2TB partition.
You can split up the hard drive real-estate into more than one partitions, if you desire.
Label the main partition name ""Macintosh HD"" as was it labeled originally on the old 1TB hard drive.
After making the partition, I went back into the upper top left UTILITIES menu and at the bottom it says to "restore from time machine backup".
I attached my TIME MACHINE Iomega external firewire hard drive to the iMac firewire port.
It then shows up in the list of drives on the machine.
Select that Time Machine backup drive, and start the backup to your new 2TB hard drive.
That process took about 7 hours to restore the full backup onto the new drive.
After the time machine restore is done, it prompts you to RESTART.
Select "restart", and let the machine boot.
DO NOT hold down any keys this time.
You DO NOT want to boot from the DVD again. So hold no keys down.
My iMac booted up looking just as it did when the old original 1TB drive was in the machine.
Eject the iMac Install DVD (you are done with that) and you should now have your iMac running with a brand new 2TB hard drive.
FINAL NOTES:
iTunes came up and worked. Everything looking fine.
BUT... for some reason when I started "MAIL", it wanted me to create new accounts. As if this was the first time I ever used MAIL on this iMAc.
No idea why.
If anyone knows why, please post.
UPDATE***
Actually i spoke too soon. "MAIL" simply imported all of my info on its own.
I did not need to re-create my account(s).