Hi: I have a Dell Inspiron 660 with 8Gb RAM and 2 HDD.  I am looking to upgrade the CPU from i3 to an i5 2380P and add a GPU.  I have the CPU and updated the BIOS but the PC doesn't work, but when I put the i3 CPU back in it boots up :(

Or, when I leave the i3 CPU in my PC and add the GPU the PC boots but nothing is showing on the screen.

What am I doing wrong?

I look forward to your responses :)