1. I assume you're asking about a MB with integrated graphics, and how much of the main memory is reserved by the graphics chip. The older MBs could, under Windows, use up to 224MB of main memory for the graphics, but I'm pretty sure Tiger uses 64MB regardless of what you're doing or how much total RAM you have. Leopard may be different, but I haven't heard anything specific.
2. You can look around the forums here for all the standard suggestions, but my vote (and that of some others) is DMS (DataMem.com)--they're not the cheapest, but their prices are still quite good, they guarantee compatibility with your Mac, their stuff is good, and they have a lifetime advance replacement warranty. They're also not fly-by-night, and I can say for certain that they do honor their warranty (I've never had a stick go bad, but I've gotten a couple of DOAs at work). Oh, and their tech support actually answers the phone--it's weird. No "Press 1 for..."
Incidentally, you can probably find benchmarks to prove me wrong or right, but I believe that on the MBs, with their dedicated graphics, that it's generally a good idea to keep the RAM in identical pairs--gives a little extra help to the relatively weak graphics system. MBPs technically benefit from it, but the real-world speed boost is maybe 2% at most, so not at all worth worrying about.
