Stick with the 2x 256MB's of Crucial. Mixing brands usually doesn't cause a problem, but I've seen instances where it does.
I've dealt with a lot of different memory modules over the past 3 years and 512MB dimms in general are more problematic then 256MB dimms. As with any memory technology, the more put onto one dimm the more difficult it is to engineer so it makes sense that it would be more prone to quirkiness.
Games that use a huge cache like Baldur's Gate II or Sacrifice will load between areas more quickly since the game will be able to store more in memory, but most games won't be make use of the extra memory and you may end up dealing with side effects, so why risk it? Unless you like trouble shooting your system more than gaming on it, it would be unwise to tempt fate when you're system has more than enough memory now.