Transfer problem (maximum number of Mbit/s)

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I have a spectrum of 900khz that is divided into channels of 4kHz each. That makes 225 channels. Then each channel can transfer 4000 symbols/sec. I will be using the QAM-256 and that allows 8 bits/symbol. That means each channel can transfer 4000*8= 32e3 bit/s. And the channels in total makes 32e3*225=7.2e6 That makes the transfer rate 7.2 Mbit/s. Is that correct? Is it impossible to go up to 24 Mbit with my current settings or am I thinking things wrong here??

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You are missing the effect of Signal-to-Noise ratio on the capacity of your channel. You can theoretically go to 24 Mbit/s if you have an SNR of > 83 dB on all your 4 kHz channels, and no interference between them... (the practical issues are significant!)

See: http://en.wikipedia.org/wiki/Shannon–Hartley_theorem

... of course, this is the best you could do if there were no other implementation losses, etc.