i have an intel core 2 dou E6550 and i want to have the best use of it capacity, how fast must my ram be?

Do you plan on overclocking? The E6550 runs at 2.33GHz / 1333MHz FSB with default clock settings of 7 x 333MHz. For best performance, the two most commonly recommended CPU:RAM frequency ratios are 1:1 or 1:2. In your case, for 1:1 ratio with the CPU running at 333MHz freq, you'd need DDR2-667 RAM, or higher rated RAM underclocked to DDR667 speed. For 1:2 you'd need DDR2-1333 RAM. If you have DDR2-800 RAM, rather than lowering the RAM speed, you can try raising the CPU frequency from 333MHz to 400MHz. But it's not as simple as changing just one setting. Numerous other BIOS settings would need to be addressed. We'd need to know the make/model of your motherboard.

Thank you so much... you comment has been so helpfull, but i have another doubt now, how does that ratio works, i would like to use the 1:2 and which motherboard would you recomend me to use for that

Which items do you actually have...just the CPU? Your system would peform better if you clocked the CPU at 7 x 400MHz with DDR2-800 at 400MHz (400:400 = 1:1). Not only would the CPU be running at 2.8GHz, but it would be running at 1600MHz FSB rather than 1333MHz FSB.