Cyclone II Max Clock Rate

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In the datasheet from what I can see the max clock is 260Mhz.

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Seems like you stopped reading the device manual at the first "MHz" value...

Actually the said 260 MHz value is a clock rate specification of Cyclone II embedded RAM.

You also have maximum core clock (internal register clocking), input and output toggle rate for different IO standards and PLL clock frequency specifications. Unfortunately your question can't be related to a particular timing specification.

To get a basic understanding of achievable FPGA performance depending on intended logic complexity, you should refer to Table 5-15 cyclone ii performance.