[GigaFrog]

Definitively, I would split my system in 2 separate (not wired) power supplies. One on the mobo, and one dedicated to electro-mechanical peripherals (hard drives, CD-ROM, fans ...).

The big advantage of this is that the noise coming fron the hard drives spinning, fans, and all other electric motors put a lot of EMI on the power lines.
When I was tuning my PIII system, ( ~ 2 years ago, time for a new system!), at the highest stable speed, it would crash on hard drive access, right when windows starts loading. I put electolytic caps on the HD connector, and it suppressed the problem.

You can even be more cost efficient by getting a real good well regulated PSU for the mobo, and an old cheap AT for the drives. You may have an AT laying around somewhere, don't you?

By wiring 2 PSUs together, you may run into problems. I wouldn't try. You might even get more ripple on the voltage lines because the 2 regulations will interfer with each other.

With a split system, you might get a higher overclock for cheaper and much simpler.

Good luck.