Sources within ATI have informed us that the current public Catalyst allows you to mix Radeon 3870 and 3850 card and make them work in Crossfire. We will check this claim as it sounds interesting and opens some new horizons to 3870 and 3850 users. This is something that Nvidia never allowed to its SLI crowd and you had to have the same cards to make them work in SLI.
It will be interesting to see what the performance benefit is when you add a second non-equal card and how much this affects performance in games.
The next driver, CrossfireX, might bring even more variables in this interesting equation.