Question:

Jordan Writes: My new 2011 iMac with 4GB of RAM is feeling a bit sluggish while running Lion and doesn’t seem much faster than my old Mac.  I noticed in Activity Monitor that some numbers in the System Memory tab are very large.  Is something wrong with my Mac?  Do I need more RAM?

Answer:

In the System Memory tab of Activity Monitor, there are two important numbers to look at when diagnosing sluggish system performance: the amount of free RAM and the amount of Swap Used. 

Free Memory and Swap Used are key values to determining RAM requirements.

Free RAM is the amount of memory that is unused by any process on the system and is ready to be assigned when needed.  If this number consistently falls below 50 MB, it is the first sign that your system needs more RAM.  To find out for sure if more RAM is the answer, look to Swap Used.  Swap Used tells the user how much space on the hard drive is currently in use because the system needed more RAM than was available.  There will usually be between 100 and 200 MB of Swap on a system with adequate amounts of RAM, but if that number grows into the gigabytes, it is a clear indicator that more RAM is required.

Good sources for additional RAM are Crucial, Other World Computing, and Ramjet.