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Seems like a reasonable price for the ram. Check around and compare before you pull the trigger. It was slightly cheaper than the best price from the site I always buy from.

Yes, I think you will see a difference. I upgraded my MacBook (2.26 Core 2 Due) from 2 to 8 Gigs, and it made a big difference. Check your page outs using a tool like iStatPro. If you get a lot after a day of typical use, then consider upgrading. Every page out is the system writing information from Ram to the hard drive's swap file.
 
2. Will I notice any difference day to day on my iMac?

I was wondering the same thing a while ago, although I have an early 09' 24" iMac with 4GB and with RAM being so cheap I thought why not.

But a very helpful user here (GGJstudios) explained to me how to use Activity Monitor, specifically "Page Outs" readings, so that by monitoring RAM usage during your normal day to day usage you'll see if more RAM would actually be beneficial or not.

In my case based on my usage it turned out that more RAM wound't have been noticeable or beneficial at all. I would suggest you do the same otherwise you might be disappointed and out a bit of cash for nothing.
 
I believe I said the same thing, although recommended a different piece of software to check the page outs. I still believe that with programs like Photoshop and Aperture, it is likely you will notice a benefit.

I upgraded my MacBook from 2 to 8 gigs, and dropped my weekly page outs from the high hundred thousands to nearly nothing most weeks. 8 gigs might have been overkill for my laptop, but I rarely feel like it is getting sluggish. I also no longer feel the need to restart it as often based on responsiveness.
 
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