Sure why not.
would it be ok to build a car pc with a ecs 945p-a atx mobo with a pentium d cpu????????????
You can, but the Pentium D is about the most power hungry chip you can find. I'd avoid it for that reason...
what cpu should i use then... with a lga775 socket.......
Look for a CPU that was manufactured using 65nm or 45nm process yet is also compatible with your MB. Those new fabbing techniques allow the processor to run more efficiently and thus use less Watts.
2006 Lancer Evolution IX MR In-Dash PC Project - WIP
Installation/Fab/Assembly (Revised v2):