Right now is a precarious time to get invested into mining. You can ether invest in FPGAs, and hope they pay off before the reward half / ASIC explosion, or you can invest in ASICs and not mine until they start shipping in an estimated 4-6 months (yes, months)
Your best bet? Pick up some cheap GPUs or some used BFL singles (mine now, trade in later).
As far as setting up a computer to run FPGAs on, any FPGA can be run on a laptop/netbook. My EeePc pulls 10watts while idle (everything disabled in the BIOS except for LAN). CGMiner uses almost no resources, and can be run on almost any hardware, regardless of specs.
As far as OS choice, it's not going to matter for power consumption that much. It's actually better to disable any hardware you don't need in the BIOS, as that will net you some power savings.