You want to spend what, at least $700 in upgrades and you are worried about your case?
OK I'll see if I can cover this for you.
First up, if you replace the MB you will need to install a fresh copy of Win 7. The copy that came with your Dell is tied to that motherboard.
So you are going to replace your CPU, MB, OS, and GPU... and also probably the PSU, to cover the GPU upgrade. So spend another $100 on a computer case that will accept the new parts.
And now that we are there, you can see it's best I think to just sell the old comp and build the new one
- An i7 860 is still a fine processor. You will not see much improvement from upgrading, unless you overclock and are doing some very intensive work.