If your building your own which is really what you are proposing then you will need a new copy of your OS. Despite what some others say it is illegal to use a Dell Disk on a non Dell system which is what you would essentially have. 
I was wandering if i get a better motherboard ( and case) for my XPS 420 desktop do I need a new Operating System???? Can I still use the one DELL shipped with the system or is that illegal use of the Microsoft Operating System???? TO TELL YOU THE TRUTH I JUST WANT TO BE ABLE TO CROSSFIRE SOME HD4890"s THANK YOU FOR ANY ANSWERS!!!!!!!!!!!!!!!!!!!!!!!!!!! MY RIG :  XPS 420 32BIT WINDOWS VISTA HOME PREMIUM INTEL CORE 2 DUO E8500 [NAME] SPEEDS ATI 4890OC 1GB GDDR5 (@ STOCK OC SPEEDS) 4gb 800mhz ram 500GB HDD & 640Gb HDD BLU-RAY DRIVE & CD/DVD DRIVE CORSAIR TX750w POWER SUPPLY  8 FANS TOTAL (not including whats on the GPU & PSU) ALL THESE PARTS WILL GO IN THE NEW CASE AND MOTHERBOARD THANK YOU    
Do I need a OS or no???