RAM Upgrade ? new iMac i5

What you are proposing (adding 2x4GB RAM modules) will speed up your machine because you’ll swap code in and out of memory more often depending upon the demands you place on the machine. Activity Monitor (Utilities) will show you how much memory you are using and indicate if more memory will help overall performance. If you are seeing that most of your memory is used frequently, consider upgrading. 12 GB is typically a very healthy amount of RAM that will keep your programs happy. 8 GB is likely sufficient for most people’s uses. Some programs will use all the memory you through at them such as graphics and video intensive programs but may not justify going beyond 12 GB.

If you install memory modules in matched pairs you will receive the maximum memory throughput (dual channel memory access paths for the iMac). If you added a single 1x8GB memory module, you will see degraded memory access because the system will only use a single channel access path.

Someone will be recommending you do not buy the extra memory from Apple but today’s prices are cheap compared to what I paid in 2009 so it still seems a reasonable option if you prefer Apple memory. If you go with another provider, just make sure they are reputable and guarantee the memory.