You can usually mix brands as long as you have the same memory type, voltage, speed and timings. If timings are not the same, you can usually correct that in the bIOS by setting the faster ones to the slower ones settings. I have mixed Mushkin, Corsair, Kingston, Crucial w/o issue but I would only mix if the matching RAM modules to what I already have was not available any more from the same vendor. If you getting different brands merely to save $, I'd advise against.
I have had issues adding Gskill to the others though. Could just be coincidence but, for example, I have a LGA 775 box upstairs for which I am building a replacement today (son no. 3's box). I had 2 GB of CAS 5 in it and adding matching modules (exact timings, voltage and speed), didn't work and w/o more Corsair's available, I had to toss the original 3 Gb and add another set of Gskills ... they were the only vendor left at the time BTW, making those modules at that late date.