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It is ill-advised to run 4 sticks in dual channel on an A64. Depending on the core being used, you will notice different effects. Venice is the most capable (unless of course you happen to have an FX 'San-Diego' or 'Toledo' which have the best memory controllers.
It will automatically default to 2T memory timings and most likely the RAM will kick down to 333mhz from 400mhz.
Nonetheless - I shall answer your question - you ought to be able to purchase any dual channel set of RAM and it should work fine with that set as you need to worry more about matching the pairs and not cross matching the different banks. i.e. 2x512MB of some value RAM should be fine of any brand but bear in mind the A64 limitations when using 4 sticks as I have said.