If you don't want to shell out for a nice switch-mode PSU..
You can always pick up a high wattage comp psu and mod it, as you yourself have suggested. However - I personally wouldn't even contemplate running it whilst still powering a comp.
BY itself.. no worries..
Here's a couple of links to help you along:
Personally though - if I had just shelled out for a nice high-end charge. I certainly wouldn't skimp on the PSU. Why risk it?
A nice PSU will have readouts for AMPs pulled, and adjustable voltage.
For all you know - something in your comp PSU may be cooking, or that extra set of tire wamers has pushed you over the edge. WHoops...
It just doesn't make sense. It's not like you buy them everyday. It's one of those important one-offs..