KNEE: You can mix the RAM, yes. However, your machine probably has 2 512MB sticks installed. If that's the case, you're going to be better off running the 2 1GB sticks (obviously).
MacBookie... to put it bluntly: you're
wrong (see definiton #2. Also see synonyms inaccurate, incorrect, and mistaken).
Yes, running in dual channel will double the RAM bandwidth (Well, on paper. It's not quite that simple, but for our purposes, it's close enough). Thing is, it makes no difference on the older (non-SR) MBPs. Why? The CPU in the older MBPs has a 64 bit wide, 667MHz bus. Which will transfer data at the same rate as a *single* 667MHz, 64 bit DIMM.
In other words: Except for the latest SR-based MBPs, running dual channel RAM on the MBP nets you an increase of, near-as-makes-no-difference, bupkis. You can take my word for it, if you like, or you can check Anand's tests here (scroll down to where it says 'page 4'):
http://www.anandtech.com/printarticle.aspx?i=2740
You might be able to measure a very small (<5%, tops) increase in performance in synthetic benchmarks (e.g. SiSoft Sandra in Windows) by running 2GB in dual channel vs. 2GB in single channel. But 3GB in single channel is going to be *significantly* faster in
every real-world scenario on an older MBP, and in almost every case on the SR MBPs, too.
And besides... There's a good chance that the i945m in the older MBPs (and the i965m in the SR machines) supports asymmetric dual channel mode. In other words, if you have say, a 1GB stick in one slot and a 2GB stick in the other, it will run in dual-channel mode while it is accessing the first 2GB of RAM, then drop to single channel for the final gig. I'm not 100% certain of that, but that's what I and a buddy of mine (who spends more time obsessing over Intel mobile chipsets than I do) recall.