Careful, not all ATX-shaped Dell supplies are ATX. (Instead, some are fry-your-Dell-if-you-use-an-off-the-shelf-part, and presumably vice-versa. This news came out around 1999 or so, no idea if or when they stopped.)
At the low end of cheap, I went with a Coolmax CX-300 on my recent Athlon 64x2 upgrade (which, being a 3800+ with integrated graphics, is not meant to be a power pig), and it's holding up fine while also being the quietest ATX PSU I've owned.
SilentPCReview seems to have one of the more comprehensive PSU test rigs out there, and
their 2006 efficiency roundup and
power distribution articles may help you characterize your needs.
