Hey Tycoon,
Yes you can mix different memory, but it's usually not recommended. Here's what you have to keep in mind:
1) All memory must run at the same voltage. If you get memory from a different brand (or even the same brand but a different model), then the memory almost certainly has different IC's (internal controllers) and therefore they may need different voltage. Just check the product page of your memory and see what voltage it uses, then find a set that takes the same voltage. Newegg's product page's show the voltage under the details tab for nearly every RAM set.
2) All memory must run at the SAME SPEED and the SAME TIMINGS. So if one set can run at high speeds and tight timings but the other cannot, then you are limited by the slower set and must run all your memory at the slower speed and/or looser timings of the weakest link. So make sure you buy a set of RAM that is capable of running the timings and speeds of your current set.