What is the make and model of the PC and the motherboard? what processor does it have?

Check the 128Mb strip and find out the CL value (it will be 2, 2.5, or 3) Depending on that, get memory that is the same. That should allow you to use the 128Mb strip as well.

Yes, I believe that you need 256Mb strips, but I would like to check that for certain. The fact that it has interpreted the 512 as a 256 indicates that they are supported. If it is a PIII I am pretty sure it will support 256Mb strips. If it is a PII we need to be careful, as quite a lot of those only support 128Mb strips.

Cheers