You can make an iMac run faster through two major ways: hardware upgrades and software fixes. Hardware upgrades are by far the most efficient way to make a computer run faster, as they add newer, faster components to your iMac. Software fixes are slightly beneficial as they optimize OS X for better performance.
Consider Upgrade Options
Upgrade your iMac's RAM memory. Newer iMacs come equipped with at least 2GB of RAM. So, if you have a newer iMac, you probably do not need to upgrade the memory. However, many older iMacs were sold with only 512MB or 1GB of RAM standard. If this is the case with your iMac, consider upgrading the RAM to 2GB or 4GB. Not only will you notice a difference in overall speed, but you will be able to run more applications simultaneously as well. Purchase RAM upgrades in matching pairs. IMacs have two RAM memory slots. Be sure to use the same sized RAM module in each slot. For example, a 1GB RAM module in each slot would give you an overall RAM memory capacity of 2GB. But a 1GB RAM module in one slot and a 2GB RAM module in the other slot would not give you a great advantage over the 1GB-1GB setup. IMacs run in dual-channel mode to better utilize RAM memory. Dual channel mode only works with matching pairs.
Consider upgrading your version of OS X. If you are running on an older OS X version such as Tiger, your iMac is outdated. The newest and current version of OS X, Snow Leopard, claims to be the fastest version of OS X yet. Upgrading to Snow Leopard should make your iMac run considerably faster.
Consider upgrading your iMac's hard drive. If the hard drive on your iMac is close to being full, this can play a role in how your iMac performs. A bigger hard drive with some extra space will cut back on seek time, allowing your iMac to find information faster. Get a hard drive with a faster RPM rate. If you have an older iMac, you may be using a 5400 RPM hard drive. Consider upgrading to a 7200 RPM drive which spins and seeks information faster. If you want to go all out, you can get a SSD (solid state drive). A solid state drive is a type of hard drive that relies on direct memory rather than a spinning disk to record and write information. SSDs are currently the fastest type of hard drive available.
Software Fixes For Speed
Repair the disk permissions on your iMac's hard drive. This can be done by using the disk utility, located at the path: Finder - Applications - Utilities - Disk Utility. As more and more applications build up on your computer, the iMac's permissions database sometimes gets confused. This can lead to apps not launching and operating at their full potential. Repairing disk permissions will fix this.
Adjust applications that start up everytime you use your iMac. This can be done within system preferences by using the path: Finder - Applications - System Preferences - Accounts - Login Items.