The system will still work, it's just that people, such as myself, prefer to have the same make etc because they know it'll work together. As long as they are DDR2 there isn't an issue.
It's not the size of the dimms. There are three things to consider when adding more memory to a system. 1)timings 2)speed and the most important is 3) whether single or dual sided on the older memory. The single sided memory simply has a row of ICs on one side only while the dual sided has a rom of chips on both sides. They won't work together.
Timings can also cause problems as well. Knowing the specifics on the memory currently in use and the new memory will help avoid problems. The speed will play some role even on newer boards wirh DDR2 where you would want the faster memory on the first channel. It will simply backclock to the slower memory there. Regarding the size of dimms the best perfomance is seen when all dimms installed are the same size. Matched pairs pretested by the manufacturer usually see a higher price too.
