How to upgrade my XPS 8900 Desktop?

    I've been using my XPS 8900 Dell Desktop for a while now and now that I've got some cash saved up I'm thinking of upgrading it. I believe some of the hardware in it is pretty good so buying a completely new PC seems like a waste to me. However, if it's more efficient to just start fresh I'm willing to do so. Also, I've got no tools so I'm also looking for recommendations on what I should buy for that.

    I mainly use my PC for gaming now but I do plan on using it for school (CAD) and personal projects (Maya), so a build that caters to both sides would be perfect.

    I've got a budget of ~1,200$ but if it could stay under 1k that would be ideal. Thanks in advanced!

Here's Dell's page on the XPS 8900.