That depends on the CPU they are used with.
IIRC, some motherboards perform better with 2x512 in dual channel than 4x256 in dual channel.
Check the reviews on Toms Hardware and Anandtech, you should be able to find the info you are looking for.
XP2800+, Abit NF7, 1GB Dual-Channel DDR333, ATI R9800PRO 128MB, TT PurePower 420W, LG DVD+-R/RW
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