You will see a big difference no matter what hard drive you choose. What is the exact spec of your iMac? Is pretty much comparing a first generation I series processor to a fourth gen. Plus I'm guessing the iMac you are looking into have more ram as well. Plus a considerably better graphics card. A good question would also be what do you do on the machine? If you are just web browsing and checking emails, then there will be hardly an noticeable difference and if you wanted a performance increase with that, then you could just buy an ssd. If you do more heavy work, then it a definite upgrade no matter the drive.