no it wont fit unless you basically cut the whole back half off the case and the mounting rack as well. i don't think anyone makes any cases that will fit a dell motherboard.
brock01 the power supply is really good go with that. but you can't just get a new case. dell uses a proprietary case format your dell motherboard is not going to fit in a regular ATX case. your going to have to seriously modify the case to make it fit correctly.
go to control panel and get to network connections, right click on your network card and go to properties, open internet protocol (TCP/IP) and make it look like the picture below.
when you go to a higher speed, it creates lots of noise in the line and that will kill speed. and that is probally what is happening here. so they are the only ones that can fix it.