2Gb to 4 Gb upgrade requires 2 x 2Gb sticks... both of your 1Gb sticks have to come out.
I've been watching the thread quoted above for $39 2Gb RAM for our MBP's. It's probably fine, but I'm just a little bit leery. When I order my RAM I'll probably get it from one of the better known online retailers, some of whom even give small rebates for the 2 1Gb sticks you'll be taking out. At the end of the day I'll be paying all of about $10 - $20 more, but I won't have to wonder if the RAM I bought is really the very best stuff. I know I'm being overly cautious, but for such a small difference it's just not worth it to me to risk compromising such a high quality, relatively expensive machine.