How much can I overclock with my 400 watt PSU?

Well, it seems all of the smarter people that use Techspot's forums are currently indisposed at the moment (and have been for 3 weeks [?]), so you get me instead.
About your inquiry, it depends on which components you will be overclocking, and by how much. I'll assume you are overclocking the CPU, RAM, and GPU, and I will assume you are targeting a "safer" overclock (meaning no overvolting).

After checking your MB's manual, I believe it does indeed support overclocking, so at first glance I think you may be in the clear with 400W; but it may also be a bit too close for comfort, especially if this system is older, which some of your components may be (no offense). However if the system is already stable at peak load using a 400W PSU, I doubt overclocking it will have a serious effect. I recommend using an online computer wattage calculator, like the one found here (which I rather like, and seems to me to be the most popular online).
Now, I had to make up for some of the parts not listed, but after looking at your MB, and using the calculator under the assumption that your build is loaded with amenities (I added a slew of fans, cold cathodes, and even pretended you are one of the few that use a PCI-e X1 slot), I came up with the following numbers, all of which reflect a CPU ONLY overclock of 15%, which puts your CPU at 3220MHz [3.22 GHz] unless I am mistaken:

Capacitor Aging = 10% = 347W
CA = 20% = 375W
CA = 40% = 429W

These numbers could be higher or lower due to my having imagined up additions to your machinery, and they DO NOT reflect the overclocked RAM or GPU, so you will need to look up their default wattage consumption and get a percentage based figure yourself. As well the calculator could always be off. You may want to consider purchasing an actual physical device to measure the demand of your components. I don't own one so I would assume something like this maybe? I know I have seen other types of devices too.
I guess you should also probably ask yourself the following:

How old is the computer?
How has it been run (i.e. at peak load all of the time, over half of the time, etc)?
What type of components are hooked up (ODDs, HDDs, fans, and so on)?

I hope all of this helps someone.
 
I doubt it's the limiting factor. Though, maybe, he toasted his computer, and that's why it's been three weeks.

You are probably right. I checked his posts to see if he had solved the issue elsewhere, but I was all too excited to handle a simple issue I felt I had a firm mental grasp on. There is something that rarely happens!

I don't think a computer even posts without sufficient power; or if it does, some components won't function properly or at all even.

...

Now I am hungry for toast. Thanks a lot.

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I don't think it would have a problem until it goes under load, then it will start drawing the most power, and then, if the power supply can't keep up, blue screen, turn off, I don't know.
 
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