Motherboards for the D8400 have always been expensive, except right after this model went "end of life". That's when brand new motherboards could be had for $25. Sorry I didn't buy one back then before the prices jumped up again.
It's your $, but all this upgrading is sounding expensive and a new 64-bit PC that can handle more than 4 GB of RAM might be a reasonable option. You can install the old hard drive as a secondary drive in a new PC and use it for additional storage space or backups, so that would make it easy to transfer personal files to a new system.
If you decide to upgrade to a Pentium 4-670, make sure it's a version of that CPU which supports NX (no execute) code. Otherwise, you'll be limited to Vista (32-bit) or Win 7 (32-bit). Win 8 requires the NX function and won't install if the CPU doesn't support NX.
The "starter" level 3.2 GHz CPU that Dell offered in the 8400 (like mine), doesn't support NX and only some of Dell's optional upgraded CPUs for the 8400 did support NX.
Since 32-bit Windows can't address more than ~3.0 - 3.5 GB of RAM, I maxed my 8400 out at 3 GB: 2x512 MB and 2x1 GB. If you buy RAM, consider crucial.com because many other brands may have the right specs but still won't work in a Dell PC. Crucial guarantees it'll work or they take it back for a full refund.
Also keep in mind that the new RAM will likely be faster than what's installed now. So if you leave the old stuff installed, the new RAM will only run at the max speed of the old RAM.