The vast majority of PSU's are externally the same size and should fit in a standard aperture. Having said, a 250w unit is quite small and presumably a few years old so it might not be standard size. I know you're an eBay king but I go along with Ritchie, use a reputable retailer like eBuyer with a proven track record for something like this.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.