Question: which one is better? 1.  two 256m.b RAMs or a  512m.b RAM? 

Answer: Two 256MB SDRAM DIMMs will outperform a single 512MB DIMM.  Each DIMM has a set number of banks, so more DIMMs means more banks.  Regardless of their size, spreading your memory out among multiple banks offers better performance because the CPU can be preparing to write to a different bank while writing to the current one.
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