You have 3 options:
- Buy a new case
- Return the graphics card
- Modify your case
Im going to assume option 2 isnt something you want to do, so that leaves us with your case.
I had this problem a few years back when first starting building systems, and after some thought, decided the hack saw was my friend. I cut away a neat section of the HDD cages to make room for the GPU and bingo, fitted in nicely with no problems.
So its up to you really, get out your wallet or get out a saw.
PS: A picture of the inside of your system will help us greatly