My development machine is heavy on memory usage. It currently only has 4GB of PC5300. I'm going to buy 8 GB of RAM, but should I take out all my existing memory and install just PC6400 or leave some of the original and stick with 5300? I don't know if I'll ever need more than 8GB, I might, I'm just wondering if the speed difference between 5300 and 6400 is worth trashing the 2 or 3 GB of old RAM I would still have room for after adding the new RAM.
If your job runs in 8gb, the faster 8gb will be a little faster.
If your job takes 9gb, the 10gb will be dramatically faster than the 8gb.
If your job takes 11gb, you'll need to test to see which is faster; it's likely but not certain that the 10gb would be faster than the 8gb.
If you're not sure, go for the larger amount of ram.