Sorry. I realize I'm late to the party and I see that you've decided to go with compatible sticks. But, for future reference, I can say that I've put extra single sticks of RAM in 8 different computers. These were sticks of RAM that I purchased as used from various individuals through an online retailer that deals with lots of used stuff. I had a problem with one of them and that was it (I think it was a compatibility issue with the SPD revision). So, it is possible. Is it a lot easier to purchase RAM in pairs? Absolutely. Do you have to? Not necessarily. With the ones I purchased for these 8 computers, the manufacturers were sometimes different than the existing RAM in the computer, but I tried to make sure that all the digits were the same. Having that said, there are some digits more important than others. The best I could tell you is, if you purchase separately, do some research on what each digit means (i.e., speed, voltage, CAS latency, SPD revision, etc.) Some numbers represent characteristics that are more important than others when it comes to compatibility.
As all have said, it's not ideal, but in a pinch, it doable - at least most of the time, given you get ones matched in the necessary compatibility characteristics.
