Its better to get the same RAM manufacturer as your existing. What is the exact model of your laptop?
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AMD Phenom II X4 955 BE, Asus M4N98TD EVO, 2x2 GB G.Skill @ 1600MHz, MSI GTX 560Ti Hawk, 500GB Western Digital Black SATA-II, Dell Ultrasharp U2311H, CM 690, Corsair VX550
@ritish: Your laptop supports 533MHz memory and is installed with 512MB RAM. I would suggest to opt for Transcend's 533MHz modules as it is preffered OEM supplier for many laptop makers (including HP, Compaq, Sony etc.).
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AMD Phenom II X4 955 BE, Asus M4N98TD EVO, 2x2 GB G.Skill @ 1600MHz, MSI GTX 560Ti Hawk, 500GB Western Digital Black SATA-II, Dell Ultrasharp U2311H, CM 690, Corsair VX550