It all depends on your upgrade path.
Since your mobo is NF3, you can take advantage of dual channel. However, dual channel on A64 platform doesn't provide as much benefit as old Athlon XPs.
2x512 will let you dual channel
1x 1024 will not (duh..).
However, if you plan on getting another one stick of 1 GB anytime soon, it will be wise to get 1GB sticks, so you don't have to get rid of 2x512 when you want 2GB total.
As far as Memory goes, different people will recommend different things. The latest RAM fads are Winbound UTT, but I don't think your Soltek NF3 can supply the voltage neccesary to use the RAM to its full potential. So, you might want to stick with TCCD chips. Search around the boards for "TCCD" and you'll bound to come up with some threads. As far as value RAM goes, ZZF/Newegg/Chiefvalue/Monarch are having price war with Corsair Value PC3200 1GB (2x512) kits. Last I checked, ChiefValue had the kits @ $86 shipped.