Just what I thought reading about it although I don't really understand why the extra RAM is not just ignored when one installs 8 GB, ie 2 4GB modules.
This seems to be a really good summary about the ins and outs of RAM used on Intel Macs: Understanding Intel Mac RAM - Mac Guides
I'm going for the 5GB otpion, 1+4, makes the most sense capacity and $$$ wise.
I was gonna upgrade my 3.1 Macbook to 6 GB of RAM. I was told that if the RAM isn't matched (i.e. 2GB + 2GB) you lose "interleaving" or something like that. Some people say that the performance increase of an additional 2GB is greater than the penalty for not having interleaving. However I seem to remember I was checking performance benchmarks and the advantage was great from 1GB to 2GB and from 2 to 4 etc. but there was only a slight difference between 4GB and 6GB so I decided on 4GB for me.
What you loose by not using matched pair is what is called "dual channel" as mentioned in broads reply.
The article I posted states:
All Intel Core Macs support dual channel memory access if matching modules are installed. The customary estimate is that this gives a 6% - 8% real world performance benefit.
What I read elsewhere is that the performance improvement gained with unmatched 2+4GB RAM (no dual channel) is greater than with 2+2GB RAM (with dual channel).
I was originally going to buy a 4GB upgrade kit, 2+2GB, but then I found a 4GB RAM module a lot cheaper than a 2+2GB kit, so I'm leaning towards that rather than a 2+2 kit provided there is not a huge downside to it.
That way I can also keep one of the original 1GB modules and end up with 5GB total which I hope is going to help vs the 2GB total RAM these MBPs came with specifically if I have a lot of applications open.
That's what I'm really trying to address - too many applications open causes the Mac to slow down until I close some applications.
Sounds to me that there is too much swapping going on between the existing 2 GB of RAM and the hard drive.