Hi D/FW
My first was my 17" Early 06 Core Duo back in Jan of 09, all went well and it only took about 30 minutes from start to finish. The 20" models have a bit more working room, but even they are cramped and harder to work on than most laptops.
Watch part 1 and 2 of Elliott's YouTube videos a couple of times and then decide if you want to go for it. While at first you might think that his video's are not up to par, I urge you to stick with him because he really did a great job on them.
http://www.youtube.com/watch?v=iVgL9u7ySUk&feature=related
http://www.youtube.com/watch?v=fI_QmBueWg4&feature=related
Your biggest concern's come from brittle cables, connectors and the risk of damaging a display or some other component connection. I'm guessing that at the time Elliott's iMac was only a year or two at max and yours is getting close to 5 years old.
There are no shortcuts. Give yourself plenty of working space, some soft towels or padding to prop things up with and maybe an extra set of steady hands to hold stuff while you work. At three years old, my camera and microphone cables were still flexible enough to just flop the front case over without disconnecting them. Take your time pealing the foil EM shielding from around the display, it's stuck real good and tears real easy. Go real slow as you begin to lift out the display checking to be sure that the cables are free and that they are not pulling or stressing the connections. At that point you need to decide if the display cables are still flexible enough to allow you to flop the display all the way over to the side or if you are going to have to disconnect it. Personally I opt for gently rolling and flopping the display over onto a folded towel or thick pad instead of disconnecting it. At that point you really have gotten beyond the real hard part! Except that you may notice there is not much room to disconnect the power, SATA and little temperature cables from the hard drive and you may actually want to remove them as you remove the hard drive from it tight little crook that it's shoe horned into. Well at that point your almost home, all that's left is to carefully pry the temperature sensor off the old hard drive and use a spot of rubber cement to re-stick it back onto the new hard drive and put it all back together.
That's a pretty good price on the Seagate! Generally I recommend WD Caviar Blue's for the all in one systems because in my experience they run cooler and quieter, but that one should work just fine.
Dennis