DDR Voltages can be changed via the BIOS quite easily. That is not an issue.
However, the difference between double sided and single sided DIMMs is not so simple. The Neo2 motherboard supports both of these. However, in order to run in dual channel mode, you must have IDENTICAL wafers. That is, the DIMMs must use the exact same chipsets and both be single or double sided. Most RAM now is double sided, so if you are getting good RAM, which I hope you are, then you should most likely have double sided. However, If you want 2GBs, I would do 4x512MB sticks, as 4 of those in dual channel mode will smash the performance of 2x1GB sticks even if they are in dual channel. The more your RAM is "spread out" the better.
Hope this helps.