Right, that's what it's called! I could only afford 512Mb of ram when I last upgraded, and remember buying two 256Mb to run them as you've described in slots 1 and 2.
I wonder if I should get a straight 1Gb this time, rather than 2x 512Mb so that I can upgrade to 2gb at some point at which point I'll get the full benefits of the dual channel?