RAM Help

So, I just got my first paycheck for my summer job, and I'm looking for some more ram for my iMac. So, I have a few questions.

1. I read the Buying RAM guide, and it said to use DDR2-667 MHz (PC2-5300 or PC2-5400) SODIMMs. I'm assuming that's correct, but can anyone link me to a decent site for this?

2. I'm not to certain about how this works, but will it be worth it to buy a two gig stick and have 3 gigs total (2 and 1) or just stick with a 1 gig (1 and 1)? Is it better to have 2 matching or just as much as possible?

Also, I have a mid 2007 imac, for any difference that might make.