is it a problem if I were to use two different brands of RAM, but same speeds? I have a 512 stick of Mushkin pc2700 in right now, but I'm seeing some good deals on Kingston pc2700 so I was thinking of getting some.
I don't think so. Check to see if the latency is the same (2.0 or 2.5). You're talking two sticks of 512MB and not a 512MB and 256MB combination, right?
As long as the modules are rated for the same speed, it shouldn't matter what brands you are using. I'm using 2 different brands* of 256 Meg PC2100 and they work great.
Your memory speed will set by the host clock no matter what stick you put in. In my case I have a 256 pc133 and 128 pc100 running at 133. My pc100 is cas 2 so I can run it 133 at cas 3. Mixed also.
It's also worth noting that many boards let you set the cas speed in the bios settings. For example, my ABIT KX7-333R lets me set Cas to 2 or 2.5, should I need too.