Here's the link to that page:
http://eshop.macsales.com/shop/turnkey/iMac_2010_27
Keep in mind that in the Imacs with 2 hard drives, one of them must be an SSD drive, which is nice, but more expensive.
Also, if you have the new 2011 version that just came out, there are issues with trying to add hard drives. The 2010 ones are okay.
If you have a 2010 one, you might think about having them add the E-Sata port rather than the second hard drive if you find the SSD's are too expensive.
If you have the 2011 one, you may just leave it alone, as the thunderbolt ports may provide ways for other solutions pretty soon.