Hyperthreading makes the difference. I think the I3 is 30-40% faster at the same clock speed then the C2D in multithreaded apps

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If the i3 laptop uses integrated graphics, but the Core2Duo laptop comes with discrete graphics. I would get the Core2Duo and the discrete graphics. Considering, that they are both priced similarly.

However, if you can get the i3 laptop with discrete graphics then I would go for that. Since, you play games and it would be more future proof.