1/ First rule of computing : The uneducated user will not be able to tell the difference between anything. Be that RAM chip speeds or RAM chip configurations. So might as well let cheapest price rule the roost.
2/ If you've got slower chips already in your computer, then the faster chips will run at the slower speed. Worth checking before you waste money on faster chips.
3/ Fewer chips is better than more unless your motherboard is capable of interleaving or other fancy stuff. So rather than, for example, 2x512MB, 1x1GB would be better.