In short: While some motherboards are a bit more flexible, basically you need two identical modules. Not just the same size, but the same speed rating and chip configuration. To be on the safe side, buy the same model twice. Most manufacturers even offer "matching pair" packages of two memory modules. The main advantage of that format is that one package is cheaper to store and ship than two, so you might save a tiny bit of money.
1 GB + 256 MB will never work in dual-channel mode, no matter how adventurous your motherboard is feeling. On the other hand, it might still be "better" than 2x 512 MB in some situations, simply because it's more memory. A bit slower, yes, but still much much
faster than swapping to the hard disk.