PSU and GPU Upgrade on XPS 8500

I want to upgrade my graphics card from the standard GPU (Nvidia GeForce GT 620) to a GeForce GTX 660 but to do this I will also need to upgrade the PSU; the standard PSU is rated as follows:

• 460W output power 
• 12V  18A

The GTX 660 specification says that the PSU needs to run minimum 450W and 12V 24A.

I was wondering if anyone knew a PSU which will run at these specs and still fit in the XPS8500 case as the case seems pretty tight on space and the standard PSU is smaller than any of the ones I can find online.

Thanks in advance