psu overload?

Depends on which videocard and CPU you have. And if it are 5400 or 7200rpm drives.

My dual-PSU PC is so powerfull that the neighbourhood dims when I turn it on

Well you're now probably running your PSU around or on its max... so you could get instability issues with another HD. But if your PSU is of good quality it could be underrated (ie. the label says 300W, but it can take up to 400W without problems) which means you have a good chance everything will work fine.

My dual-PSU PC is so powerfull that the neighbourhood dims when I turn it on

There are mathematical ways to do this, but they aren't always as reliable as we'd like. As SVol pointed out your supply may be underrated or it may be overrated, some devices may draw more or less current than specified, and so on.

About the best way is to install monitoring software to keep an eye on the voltages and give you a warning if there are any problems. If the alarms start going off, then you upgrade your supply.

The one I use is here:
<A HREF="http://mbprobe.livewiredev.com/" target="_new">http://mbprobe.livewiredev.com/</A>

I prefer it to more complex ones, it's small, sits in your task bar and it's free...



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