If you have never worked on a Mac or iMac before, my advice would to take this into an Apple store or an Apple authorized service center and have them install the proper hard drive for you, instead of you doing this yourself and risk "bricking" your iMac.
Get an estimate of cost and just have certified Apple Tech do the installation. I shouldn't take long for them to do this.
The reason I advise having this done by Apple service techs is if you open this iMac up and work on it yourself and you fail to get your iMac working again, you will not be able to bring to Apple, directly, any longer. They will not do the repairs or installations needed to your iMac. You will be forced to take it to a third party Apple authorized service center for repairs from now on if you attempt to do this drive replacement, yourself.
Tearing down and reassembling an iMac is not for a "newbie" or novice computer user/ upgrader.
iMacs do not have any easily serviceable or user upgradeable parts inside.
I have a lot of Mac upgrading experience and the newer flat style iMacs (and the much older eMacs) are about the only Macs I will not attempt to try and work on.
Too many screws and screw sizes. Too many really fiddly and fragile connections. Just too many variables of things that can either break or forget or hook up improperly. The 2005 and newer flat screen style generations of iMacs are just complicated and finicky puzzles to work on.
Thanks a lot MichelPM for the wise advice!!! I am aware of the risks involved,I really am!But I have dealt with fiddly connections inside my Macs,at home,since my Power PC 8100,back in the 90s.I am,by no means, an Apple Tech,but I do have enough structure,memory and nerves to pull this off.Besides,it's my only solution cuz the prices of Mac sevice in SouthWest France(where I live) are deadly,insanely unreasonable. So,yes,I will have to take care not ot ''brick'' my Imac,as you say.
But,my question really was do I have to change any jumpers on a SATA III drive before I install it.Or anything else ... ??? I can't find anywhere a SATA I or II drives? You could,perhaps,if you have the time,recommend a particular model to me( I'm not after huge capacities,cuz I have external FW800 drives for storage) but 500GB,750GB or the like.1TB max.My iMac model is MA878LL (24'',Mid 2007).
I would be most grateful,