Not sure if your board supports dual channel, but if it does do not take advantage of it. There is no noticable performance increase by using dual channel memory with an Athlon CPU. None. I can tell you personally because I went from single to dual channel.
The Athlon's bus path is 64-bit; dual channel is 128-bit. Save yourself some cash and buy one stick of PC2700 512MB or 1GB memory.
Actually to be on the safe side, if you can get PC3200 (sometimes it's cheaper because of popularity and things of that nature), that would be good. Sometimes DIMMs and certain motherboards don't like each other too much, and when you run it at the typical speed and timings (the memory that is), it's unstable. Having one step up helps, because you can then decrease the timings and speed and get stable performance.
My A7N8X Deluxe hates my Kingston 512MB DIMM at PC2700 speeds. Thankfully my CPU's FSB is 266MHz, so I only need PC2100 speed.