So, it's DDR2 and LGA775, no desire to overclock... I'd reccommend something with a 915 chipset, cheap, reliable, but remember: no room to expand for the future.
Here's an ABIT GD8 Pro that's about $50 shipped after rebate (if you do rebates):
As long as you use the OEM CDKey that came with the Dell disk to begin with, you should be fine. AFAIK, it's only the CD Key that's tied to the installation disk type... I tried installing a retail XP disk with an unused Dell key once and it would not accept it.