The first thing is to check to see if the computer/BIOS will support 1Gb or better chips. Often the BIOS will not recognize the large chips - a BIOS update will often solve this.
The other problem is the power applied to the memory slots may not be sufficient for the larger chips. We ran into that problem with some Compaq machines way-back-when.
Another problem is that some machines require matched memory - you cannot mis-match memory sizes in the slots. We had that problem with Gateway and early Dells. Although you can use only one chip, if there are two they MUST be the same.
I doubt the performance increase between 2 x 1Gb, 2Gb +256 Mb and 2Gb (1 chip) will be noticeable if the machine will boot.