If you want to run in dual channel and you want to buy two 512MB sticks, you will have to leave it out. I belive you can run with different size sticks if you do the following:
Memory Slot 1 - 256 MB
Memory Slot 2 - 256 MB
This would be the first channel
Memory Slot 3 - 512 MB
This would be the second channel.
The channels must balance in terms of the size of the memory.
As for mixing memory, I have a question about that myself. I have two sticks of 256 MB high performance Corsair memory and I was wondering if I could pair those with a stick of 512MB Kingston HyperX memory to run in dual channel. I dunno.