interesting question... athlons/durons need about 1.1x to 1.2x more mhz to match a p3, celerons are crap... and p3s you can go dual, but then you'll have slightly lower times per wu. assume that case, ram, hdd, nic, etc, are the same price for each combination. i815s and i810s don't need video of course.
ooo... new pricewatch...
dual p3s - msi 694d mobo $120 + (2) 700e ($145) $290 = $410. 2 work units in 5 hours = 9.6 per day. $42/wu standard oc is to 933 and does a v3 wu in 3.5 hours each $410/13 wu/day = $32/wu
duron 650 $45 + msi kt7pro2 $115 = $160 4 per day or $40/wu. oc to 950 and get 6.4/day for $25/wu
that duron is looking pretty cheap, either way. non overclocked it only have a $2 lead over the p3, though. since you're building 3 rigs, may be best to go durons. if you can thrash it higher than my estimated oc of 950 (overclockers had a low average due to lots of people reporting locked multipliers) it gets even better.
of course, these results are based on the v3 client, not 3.03, which takes much longer. tack on $100 for hdd, $150 for ram and case.
dual p3 700s $69/wu
oc'd to 933 $48/wu
duron 650 $102/wu
oc'd to 950 $64/wu
then the dual p3s look good at the sure-fire 700, and look much better at 933.
heh, just looked at what an athlon thrashed up to about 1200 can do... 8.25 wus per day, $175 for the processor, $115 for the mobo
$65.5/wu
so it doesn't seem to touch the mighty dual 933, either.
disclaimer: my numbers could be wrong, they're mostly guestimates based on reasonable pricewatch pricing, overclockers.com oc'ing averages, and TLC's benchmarking. and, as always, your mileage may vary.
EDIT: $75 for a celeron 633, overclock to 950+ pretty easily, i think. msi 815 pro $95 so $60/wu not too shabby.