No, that motherboard is an ATX form factor motherboard and thus incompatible (not to mention too large) to install on that system.
It is often hard to fit an after market motherboard into dell computers, you would have to check the dell website.
I'd probably go for a Sony VAIO or a Dell Inspirion.
A few places where one can purchase a Dell Inspirion on the Internet, include the official Dell website, the Best Buy website, and the website Amazon.
One can find reviews on Dell Inspirion 640m on websites like notebookreview or on the official Dell website. One can also find reviews in computer magazines.
Inspirion Dell laptops can be purchased at the official Dell website, Future Shop, Best Buy, Best Byte, Walmart, The Source, Tiger Direct, Amazon, and Ebay.
No sorry the Dell Optiplex 780 can't support a i7, it would have to be a 1155, 1156, 1366 socket compatible motherboard, but your options would be a Intel Core 2 Quad like a Q6700, upwards. Your Dell has a 775 socket.
Most Dell desktop computers will have a microATX form factor.
Dell inspirion is better,especially for first time users its better to understand and navigate through.
Yes. You can upgrade your Dell XPS 400 Intel Pentium D to Intel core i7.
The Intel Xeon does not exactly make parts for Dell Computers. Intel Xeon is a technology and it is used in several models of Dell computers. Along with several other models, the Dell Precision T3500 contains the Intel Xeon technology.
The most popular of the laptops that Dell sells in the Inspirion which comes in white. They also offer a mini in white. The other one that Dell offers is a Studio XPS which is in white.
The Dell Optiplex GX620 SFF uses an F8101/0F8101 type motherboard.