Extending works better if you have a PSU that has remote sense lines for all the positive voltages (and you wire correctly to preserve than awesome function).
"Remote sense?" you say. Okay, maybe
you didn't say that, but someone out there did.
As part of the voltage regulation, PSUs monitor their voltage outputs and adjust as necessary. But with the larges currents running to the motherboard and the small wiring resistance, that voltages at the mobo don't exactly match the voltages inside the PSU.
Standard on ATX power supplies is a thinner wire attached to the +3.3 V lines. It doesn't carry power to the ATX connector, instead it allow the PSU to measure the voltage at the ATX connector. The PSU doesn't output 3.3 volts -- it output whatever is needed to give 3.3
at the connector.
Newer, better supplies have remote sense on +5 and +12 volts, too. If you have one of those, you can make your wires really long and still have the proper voltages at the mobo, though voltages on your hard drive and floppy connectors will be a bit higher than normal.
And, if you're really cool, those remote sense lines make for an easy way to tweak your supply voltages.
