p.s. as a further point of reference, take a look at this article - it's not regarding your model MacBook but rather a newer generation unibody MacBook Pro that DOES support interleaving and prefers matched DIMM modules.
you'll see that real world testing showed almost negligible performance in that model MB Pro with mismatched 2GB+4GB modules, even though interleaving is supported.
so even if your Macbook C2D supports interleaving (again, I don't know if it does, or not), then if you are running memory-hungry apps like Photoshop etc, I think you'll be far more likely to notice an additional 2GB of RAM in day to day usage than noticing any performance difference due to interleaving. As you've clarified, your MB is a "Early 2009" version, which is a model that 6GB has been tested to work ok.