MacBook RAM question

I have a 2007 white Macbook that runs at a max RAM of 3 GB at 667.
Until recently I had 1GB at 667 and 2 GB at 800.
System profile showed 3 GB at 667.
I have now installed two new 667 2 GB sims and system profile shows 4GB but uses 3 GB. To be honest I am not sure if the Macbook runs any better than before.
I ran some memtests on the the new setup and there were some mismatching errors.
Any advice much appreciated.
TIA

I would not expect to see any performance improvement after going from 3GB to 3GB. 3 is all the machine will address.

I believe two matched pairs of sims running at the correct speed of 667 even if only using 3+ of the 4 GB available would theoretically swap data and run better than mis matched sims ie. 1GB & 2 GB at 667 & 800....
Perhaps I just wasted my money

I have now ran various memtest and benchmark programs using different RAM configurations and my MacBook performs noticeably better with a matched pair or DDR2 2GB 667. No question about it.