I'd hate to be the bearer of bad news, but its not.
If you look at this link, you'll see you want PC2-6400, 200-pin, 800MHz DDR2 SDRAM.
http://support.apple.com/kb/HT1423#2
That RAM you linked will work for an early 2009 iMac, but not early 2008 (or at least is not what is recommended by Apple). If you scroll down to the product description of the RAM you linked, you will also see that it mentions early 2009 iMac.
Here's a link filtered to the RAM you're looking for.
http://www.newegg.com/Product/Produ...06176 600000401&IsNodeId=1&name=8GB (2 x 4GB)
In case you didn't know, an iMac (which is essentially a laptop that doesn't fold up and has a large screen) uses laptop RAM (and laptop hard drives, if you ever go that route).
In case the link to Newegg doesn't keep have my search results, here's what I clicked under Computer Hardware -> Memory.
Laptop Memory
Speed : DDR2 800 (PC2 6400)
Type : 200-Pin DDR2 SO-DIMM
Capacity : 8GB (2 x 4GB)
Also, don't let the "Mac Memory" section trick you. There is no such thing as "Mac Memory" or "PC Memory". I'm not sure if they are just trying to sell their products for more money, or just assure customers that there is a Mac that uses this RAM, but either way, there are a lot less results if you limit yourself to RAM that has been labeled "Mac Memory".
Also, (if it makes you feel any better) I replaced the RAM in my Macbook Pro and didn't use "Mac Memory", but just what ever was cheapest of the exact type Apple said to use (which happened to be G.Skill).