Get the 1 x 512MB now, and a second later as needed. The dual-channel DDR doesn't provide any significant speed increase in most applications, and certainly less than the 200mhz FSB you should be able to run at spec'd timings vs at 166mhz with the PC2700. I recently (2 months ago) purchased 2x256MB of Samsung Original True PC2700 and now wish I bought either 1 x 512MB or the full gig up front. Of course, the sticks only cost me ~$25 each, but thats pending $60 in rebates.
Chiz