Most mobo's will allow you to mix 256MB and 512MB sticks aslong as you even out the channels and they are configured identical.
A possible config for DC-mode:
slot A1: Corsair 512MB CL2
slot A2: Infineon 256MB CL3
slot B1: Corsair 512MB CL2
slot B2: Infineon 256MB CL3
(Slots Ax belonging to the first channel and Bx to the other.)
This will put 768MB on each channel in identical config and allow the mobo to operate in dual channel mode.
However the slower CL3 timing will be used and like others have said before:
Mixing different brands/speeds/sizes may cause stability problems.