Generally you wont have a problem with 512mb sticks, or with micing brands.
However, you can run into problems if you closk the memoy to a lower CAS setting, and one of the sticks is incapable of running at that speed.
It was with SDRAM, but i had a system that was totally unstable until I set it back to CAS3. Seems the new stick of RAM that I inherited from a friend didnt like CAS2.
If you're not a tweaker or an overclocker, you usually wont have these problems though!