A one giga DDR 400MHz considered quite good, don't under estimate the speed vs memory, I would go for the 1 giga 400Mhz.
I have 512 mb ram, but their frequency is 400MHz therefore I don't seem to have any lose in performance nor need in even more memory (although an extra 512 would be great
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Pc2100 is 333 MHz? (Correct me if I'm wrong) I would leave that alone and as I said, go for the one giga 400 MHz. :thumbup:
If your brother doesn't mind performance...get the 333Mhz cheap (used ... I guess) memory.