Another vote for the celeron. An lga 775 board is more likely to have DDR2, SATA and PCIe then a 478 I would think. (struggling to find facts to back this up though)
Since this PC is being made from spares, I should check interfaces for HDDs, graphics cards and what RAM you have most of before deciding on one or t'other.
Yup, I found a old Biostar, with DDR2 and PCI-e 1.0 LGA775. So I'll use that one. Thanks for all the advice.