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You choose VCCO according to the requirements of whatever external devices you've connected to the FPGA I/O pins.
Here's a relevant section from chapter "SelectIO Resources" in the Virtex 4 User Guide UG070:
Output Drive Source Voltage (VCCO) Pins
Many of the low-voltage I/O standards supported by Virtex-4 devices require a different output drive voltage (VCCO). As a result, each device often supports multiple output drive source voltages.
Output buffers within a given VCCO bank must share the same output drive source voltage. The following input buffers use the VCCO voltage: LVTTL, LVCMOS, PCI, LVDCI and other DCI standards.