I need some help about what memory should I buy for my computer.

Specs of my PC:
Intel Core 2 Duo E6300 1,86Ghz 2Mb (FSB1066) - Socket 775
MSI 975X Platinium Power Up Edition - Socket 775
HD Western Digital Caviar SE16 250GB SerialATA2 16MB buffer
Case Thermaltake Aguila VD1000BWS Window Black.
Power Supply 550w Antec TruePower 2.0 TrueControl II 550.

Here in my country (Argentina), I have two options for the same price.

DDR2 2GB (2x1GB Dual Channel kit) PC-4200 533 Mhz OCZ Value Pro
Model: OCZ25332048VPDC-K
Timming: 4-4-4-8

OR someone of this:

DDR2 1GB PC-6400 800 Mhz OCZ (2x512MB Dual Channel kit) Gold GX XTC
Model: OCZ2G8001GK
Timming: 5-5-5-12

DDR2 1GB (2x512MB Dual Channel kit) PC-6400 800 Mhz OCZ Special Ops Edition
Model: OCZ2SOE8001GK
Timming: 5-5-5-12

DDR2 1GB PC-6400 800 Mhz G.Skill (2x512MB Dual Channel kit)
Model: F2-6400CL4D-1GBPK
Timming: 4-4-4-12

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Can somebody tell me what option is better for my Computer?
2GB(2x1gb) 533Mhz or 1GB(2x512mb) 800Mhz?

I will use my computer for good games, like Oblivion, Far Cry, etc. Of course I will use for applications and others things, but I want the best configuration for games.

Thanks in advance.

Nobody can help me?