Yes, there is a huge difference. Especially with a PowerBook.
The max RAM (stated by Apple) is 1 GB in you machine. Now, there are two ways to have 512 MB of RAM. One was is to fill both slots with 256 MB, leaving zero slots open. The other way is to get 512 on one chip, leaving the other slot open. What is the difference?
Next year, you want to upgrade. How do you do it? In the machine you have 2 256's in you must remove one before replacing it with a 512. Although you think you might get 1 GB in total, because you are adding 512 to your 512, you actually end up with 768 MB (512-256+512=768). How much sense does it make to buy a 512 MB chip only to upgrade your actual RAM by 256?
On the other hand, when you have 512 in one DIMM, upgrading is easy. All you do in snap in the new RAM, because you have 1 slot open.
So, my recommendation is to get the RAM in 1 DIMM for future expandability. Sure it will cost a little more (especially at the Apple Store!) but it is well worth it. At this point in time, with memory being so cheap, nobody should ever buy a DIMM less than 512 in my opinion.
Good luck.