Nvidia upgrade??

Hi,

Just after a simple, straight forward answer guys!

I have a Dell XPS 8500 (Early 2012) running 8.1 pro - i7-3770 CPU with 24gb RAM & want to upgrade the GPU.

Presently it has an Nvidia GeForce GTX 660 & all I want to know is what MAXIMUM size Nvidia Video card I can have fitted? I hear there are physical space restrictions so I don't want to go out & buy any! (not being computer savvy), so just want the max available in the space available.

Also what size power unit would I need to run it?

Any help would be massively appreciated..

Thanks

Rick