Early-2008 iMac - RAM upgrade options?

Use activities monitor located at /Applications/Utilities/

check memory pressure in memory tab if its green you don't necessarily need more ram

check swap used, if it's 0 bytes then also you don't need more ram

Note: Check these two while having opened 2-3 applications (or your usual workload)

That model officially supports 4GB RAM. However, with newer, larger RAM modules introduced since Apple engraved that model's specs in stone will allow you to achieve a total of 6GB RAM.

The "catches" with teh newer module are two-fold:

1) Only a handful of RAM vendors have the proper stick to get you to 6GB. The only vendor I trust for that module is OWC:

http://eshop.macsales.com/item/Other%20World%20Computing/6400DDR2S4GB/

2) Normally, as the capacity of the module goes up, the price goes down slightly. Not with the special 4GB module you need to get to 6GB; it goes up a LOT. The following chart shows the cost per GB of the above vendor's "kits" required to achieve 4GB and 6GB in your model where youi to start as the base 2GB RAM:

Your current 4GB RAM will go much farther if you do a few simple things:

1) Don't run third-party anti-virus, internet security, or so-called "cleaning/tune-up/optimizations" utilities.

2) Keep a minimum of items in your user log-in items (in System Preferences > Users & Groups > [your admin acct) > Login Items tab)

3) Google apps can be taxing on RAM-constrained older systems.

4) Make sure yo udon;t have adware: Remove unwanted adware that displays pop-up ads and graphics on your Mac - Apple Support