i got a Asus P6T, X58 chipset board 2 years ago and it had 3x2gb of Kingston valueram 1333mhz.
now im running very ram heavy games and HD video editing. planning to make it 12gb, adding another 3x2gb in there however i can't find any kingston value ram of the same model anymore. most likely i'd end up with other model of kingston ram, corsair or g-skill or something. will the board take 12gb like that?
NOTE: i've heard from internet somewhere people saying Asus P6T don't take 6 sticks very well, especially now it will be different brand/model and will have problems!!!
There are always the odd exception, but as a general rule, yes this will be fine.
There is really only 3-4 companies making DRAM chips, so there's a chance that the chips you get on the new RAM will be the same as on the old RAM. What you want to pay attention to however, is the various performance metrics. The closer those match the better. Even if the latency timings are different it should still work, you just pay more for lower latency on some RAM that you won't be able to take advantage of.