More then likely you will never need 8 gb of ram and if you ever do, you won't need it on an A64 build. Maybe in the future but for now even more then 2gb is kind of an overkill. Don't worry about that.
To answer your question, X2s support a maximum of 4gb. While opterons (not sure) 6gb max. << Can someone clarify that ?
To answer your question, X2s support a maximum of 4gb. While opterons (not sure) 6gb max. << Can someone clarify that ?