Yeah, Thats how I would set it up too. You'd have to read your mobo manual to see if it will run in dual channel.
It will most likely not, but some boards support dual channel like this.
0- 1 gb
1- 512 mb
2 -- empty
3 - 512 mb
(channel 0 = (0+2)= 1 gb and channel 1 = (1+3)= 1 gb)
either way, the more ram the better. I'd stay with all 3 sticks