I have an old comp which had 2 modules of 128mb up until today when i switched them to 2 * 256mb. everything works fine and i can feel the change but i was wondering if i could get more by using all four modules. Is it possible to use 2 sets of different size? I have also notices that there are 2 pairs of slots, does it matter which ones i use? my board is Intel Rock Lake D865PERL by the way.
More aboutmixing memory modules
Here's a link to the manual for your motherboard. On page 34 it shows how to install memory and what the allowable combinations are. You can use all 4 of your memory modules as long as you install them in accordance with the manual.