Need a new motherboard

I dont know much about what cpu works with what mobo. Does all lga775 socket cpus work with all lga775 mobos do I have to get a mobo with the right cpu type like intel pentium 4 prescott/celeron D or Intel Pentium EE/Pentium D/Pentium 4 HT I dont get it  

I've worked on quite a few Dells and replacing the motherboard with one other than from Dell usually leads to problems with front panel connectors, fans, etc., as Dell does some proprietary things with their hardware. Most of these things can be worked around with some rewiring and new connectors, or changing the case, but it does end up being quite a pain.

Iam just looking to keep the cpu from the 8400 and rebuild the comp with a new case and power supply.

What kind of features are you looking for? What will you use the computer for? ATX or mATX? Give some info and we can help.