But the real question is, "Why?" That's a crappy, overrated PSU. A 380W Antec Earthwatts would be a much better choice, and they're only $40 right now.
And no, you don't want Athena Power either. If you're limited to the smaller size, here's a unit by FSP that is efficient, stable, and has gotten good reviews at jonnyguru and hardwaresecrets: http://www.newegg.com/Product/Product.aspx?Item=N82E16817104075
Looking at the manual here: http://support.dell.com/support/edocs/systems/dim5150/en/om/WD846A02.pdf
it looks like a standard ATX size, but the screw placement looks "off." I can't help but wonder if, as old as this one is, it may be one of the last systems Dell built that used proprietary connectors. If that's the case, you would need to contact Dell for a replacement (and likely pay through the nose).