Intel boards are more tolerant of "asymetrical" RAM quantities than most boards. Asus boards have a reputation of being picky about the RAM it chooses to boot with.
Dual channel RAM with different size sticks is doable in some cases. It's called "dual channel interleaved mode". What happens is this, with 1.5GB installed, the lower 512MB from each channel will run as dual channel. Then the upper 512 (1GB stick) will run as single channel.
Will it work for you? I have no idea. You would probably spend more money buying 1GB sticks on 2 occasions than just jumping in and buying a 2GB dual channel kit from the jump. DDR2 is way cheap now.
So, I recommend buying a 2GB kit of Asus certified RAM from the git go, it would make your life simpler, much simpler. Asus should have a listing on their website as to which RAM is compatible with which board, I suggest you visit them before doing anything.