Since you dont have a transparent shader (glass or something using alpha like polygon hair) you dont need the Light Throught Transparency turned on.

The settings that I talk about is for better image definition: if you set to 4 (for example) Carrara will use 4 pixels for every ray traced... so it will do an aproximation to the real thing. Setting to 1 (or 0,5) Carrara can render a pixel per ray...so more definition and less "dust" in the final image.
Sometime we need to increase the Photon Map Accuracy to a better value, this will cleanup the image more and make your render time larger.

The idea here is to try the best settings with reduced image versions so you can more quicly see what is happening with every adjustments, maybe something like 320X240...then after this preview if everything is right jus save your scene and let the computer work in the final image with final resolution.

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