2x256 versus 2x512, Post your observation

My observations:
About 10% gain in PCMark2002 (memory).
About 50-55% gain in Aida32 "write" mark.
No difference in Sandra mark.
No difference in 3DMark2001SE.

I'll add one more -
difference is significant in MemTest86 as well though I cannot estimate approximately how much.

Which one do you get the gain in?

2x512.
These test programs are dead foolish. Aida32 for example does not say whether the bench is done in 2x256 or 2x512 or something else. Sandra does not say what processor speed was used and that matters a great deal.
That leaves you to guess how good your system really is.

Another thing 4x256 is not equal to 2x512, that might surprise some as well.