Does more RAM on Mac really improve performance?

I'm coming from a PC, loaded with a Core 2 Quad CPU and 8GB of DDR2 RAM. I was running Premiere CS3.

I'm new to Mac so I'm not sure if this will help performance: Will increasing my 21.5" Core 2 Duo iMac's memory from 4GB (DDR3) to 8GB improve performance of Premiere CS4 significantly? I am not impressed with Premiere as it is now. The iMac is the newest one as of this post.

Yes! Mac OSX loves RAM and so does Premiere. Max out the memory if performance is important to you.

Do you "need" 8GB or 16GB? As others suggested, best thing to do is look at memory usage during actual usage. Without upgrading, it might be hard to tell but give it shot. A couple nice articles about OSX memory and performance might be worth reading: Problems with lack of memory in OSX & OSX Performance Tuning

I couldn't find a definitive answer, so please verify, but I think you can add 2x4GB SO-DIMMS giving yourself 12GB. That would let you upgrade without having to throw away your upgrade later if you do upgrade to 16GB. Unfortunately, there is currently a significant price premium for the 4GB SO-DIMMS.

Lastly, if you have the GeForce 9400M version of the 21.5" iMac, chances are good you'll see a large boost from 8GB since your video is sucking up system memory.

Open up Activity Monitor (it's in Applications/Utilities) - select the Memory option - run the program you're interested in (Premiere) - stress it as much as you can. If you still have a green segment (unused RAM) then you don't need to add any more RAM.