What is most updated Equipment that can be used in a Dell XPS400?

I found a picture of your board, and it's a btx format. There are a few btx boards in the uk which will work with the newer wolfdale cpus, such as the e5400. Can't recommend a vender. I doubt your old motherboard will work with any new core2 duo cpus. If you don't find any g31 or g41 btx boards in the usa or uk, look for the g965 or g33 chipset, which will run the "e" 6xxx core2 duos, and maybe the famous quad q6600. Your ram is pc4200, which may also work, but I'm not sure. You'll just have to try it. It's not possible to use an atx or micro atx board in your dell btx case.

post updated.

sry for being late but Thank you I'll check this info out & see what I come up with. IBB