Same size memory chips = better performance?

Essentially yes, the slots will run best in 'dual channel mode' this means;

The blocklets of data will be written to memory in a similar way to a striped raid hard drive setup. Data of the same origin, or next in line will be written simultaneous to each memory stick. The idea being is CPU fully utilises the bandwidth of each memory stick when it access the data it needs.

The CPU has a harder time if those two places have different sizes, meaning more blocklets of data are stored on one than the other thus not using the full bandwidth of the smaller stick of memory!

My advise; sell your 512 when the time comes and go to Crucial, for instance as they sell Apple compliant memory, and get 2x1024 DDR2 667

Andrew is correct, however the gain is very small and you will not notice it. The biggest gain for you will be adding the gig of RAM.

It's how the memory is allocated to each processor core. Since there are 2 processors, 2 equal size ram chips give each equal memory to avoid bottlenecking. If one core needs more memory, the OS can allocate it. If the OS gives each core the same amount of intensive processes while one core has 512 MB more ram than the other, it will run slower and can bottleneck the system slightly.

CPU has nothing to do with allocating memory, thats the memory controllers problem which on the Intel line is a separate chip on the motherboard.

The CPU has a dynamic L2 cache system that is managed by the CPU, but this is an entirely different area of the system. The main system memory store is what is being questioned here.

Usually on a Windozes system you will notice a bigger gain when running in dual channel. On Mac's you don't notice that much at all, why? They are using the same Intel chipset and Intel CPU.

It's due to the memory management that windows uses. windows has terrible memory management and tends to use nearly double the amount of memory to keep it running in the first place.
The difference is more to do with the architecture of the system, the drivers are tuned to make better use of dual channel than OSX is.

The reasons are quite simple, windows and their various driver and company counter-parts have been making use of dual channel longer than Apple has had the technology. Apple now has the new Intel hardware to work with, they just need to develop better software and hardware interaction mechanisms

This gives them the edge for the time being, now Apple has Intel as its main CPU and motherboard supplier we should see OSX and Intel work closely to make the very most of Intel's archiecture.

In essence we should see Intel and Apple pull ahead of the competitors as getting an exclusive deal such that Apple has with Intel they should have more information about whats coming next to make the very most of the technology!