Impossible to give specific advice without knowing your iMac's specs and what you use it for.
But to improve performance, you will want adequate free space on your hard drive, adequate RAM, limit the number of apps open at one time.
If you want to purchase a new HDD, look for 7200RPM and be sure you will give yourself at least 20% free space. As a general rule of thumb, at least 4GB of RAM would be recommended too. But that's not a steadfast rule or anything.