Depends. Is your motherboard capable of dual channel operation? If so, 2
256s will give a bit better performance. Using 1 512 will leave more room
for expansion.
Depends. If all you need is 512, and your system is capable of Dual
Channels support, then two might be better. If your system doesn't
support it, then you may be better off getting one 512.
Always get as _FEW_ sticks as possible!
If you must use more than one, make them similar, if convenient and
possible.
Do avoid, to considerable lengths, using more than two sticks.
Of coarse, if you want to run dual channel you need two, and they must
be similar.
Unless you're doing a 800MHz fsb P4 on a dual channel mobo, the
correct answer to your question should be one 512.