Is there a typo or something I'm missing here? Your choices are:
1x 256 MB PC2700 DDR Sdram or
2x 128 MB PC2100 DDR Sdram
Either way you have 256 MB total correct?
Your board is not an nForce 2 chipset board, so having multiple sticks of ram gains you no performance increase. (It's a Via KT333 board if memory serves.)
If this is true, the single PC2700 256 MB stick would be the better option for a few reasons. First, it would allow you to use a 333 FSB Athlon XP (2500+, 2700+, 2800+, 3000+) without switching out your ram. (The 2500+, when available looks to be a great deal. It should perform very well for the money AND be quite overclockable - a 200 MHz + overclock should be easy. ) Second, if you use a 266 MHz FSB Athlon XP (all the others), you can overclock the FSB if you choose and keep the memory within its specifications. Third, you have more memory slots remaining free.
Hopefully I correctly interpreted the question...