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#11
I suggest that you replace both SODIMMs at the same time, to avoid compatibility issues caused by mixing memory with different specs. You can get a small price break for buying a pair:
Newegg.com - CORSAIR 4GB (2 x 2GB) 200-Pin DDR2 SO-DIMM DDR2 667 (PC2 5300) Dual Channel Kit Laptop Memory Model VS4GSDSKIT667D2 - Laptop Memory
You may not get the full capacity of 4GB of RAM in Win7 X86 (32 bit, widely know addressing limitation), but you'll max out the RAM.