I've decided to upgrade my 2010 27" i7 iMac with a 500GB Samsung 840 SSD. After being wholly impressed by the performance of the Samsung 830 SSD in the MacBook Pro, I want the same for the iMac.
Samsung Electronics Samsung 840 Series Solid State Drive (SSD) 500 GB SATAIII 2.5-Inch MZ-7TD500BW
Although the iMac only has a SATA II interface and the 840 a SATA III, the 3Gbits/s speed of the SATA II will still be a dramatic increase in speed over the traditional hard drive.
I've also ordered a special toolkit from iFixit.com for opening up the iMac.
With Amazon Prime, I have free 2-day shipping, so provided the order and shipping goes well, I ought to have everything I need by the weekend and will be working on the upgrade then.
Download the pdf version with pictures here.