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Tom's Hardware : Do You Really Need More Than 6 GB Of RAM?

Not much has changed since 4 GB of RAM became the “sweet spot” for performance and price in the enthusiast market. While 32-bit operating systems previously limited those 4 GB configurations to around 3 GB of useful memory space, today's test shows that 3 GB is still usually enough.

Which is good, as 32-bit versions of popular operating systems can't actually address much more than that anyway. They do say you might go to double that if you have a 64-bit OS which can take advantage of it, but more on some future-proofing theory than anything else. Note, however, that it's far from costless,

Every time we doubled memory capacity, idle power consumption increased by around 10 W. Using the Sandra Memory Bandwidth benchmark, load power consumption for the entire system increased by around 10% for each increase in memory capacity.