Personally, having 1 or 2 modules, speedwize, should not make any difference. The memory controler operate at the same time on all the modules. The time it wait from the modules doesn't 'add-up'.
Where it will come handy later is if you wish to add memory. If you have only 2 slots for example, and you have 2 modules of 256MB, then, adding memory will involve loosing a 256M module. So, if you wish to upgrade to 1GB, and add another 512MB, if all slots are took, you'll have to remove one 256MB module, ence ending up with 768MB (256MB + 512MB). If you start with a single 512MB, then, it is easy to add more memory.
Moreover, if it's cheaper to buy 1 512MB than 2 512MB, then the choice is obvious.
One thinking you could have is if you upgrade your system later, it is easy to take a single 512MB memory to add to your new board but in practice, I never seen memory technologies which last long enough. Ex: I do have nice DDR-266 in my system. This was top of the line when I baught it. But even though I have 512MB, I would not transfer it in a new system that support DDR 400 or 533!