Dell Xps 8100 Upgrade GPU

Hi everyone, I need some help with a graphics card upgrade for my dell studio xps 8100. I've had this computer for a few years.
I know a little about computers but my main question is what is the best graphics card I could get? I just don't know what would be compatible with my computer or how to tell.

My system:

  • Intel core i7 - 860 quad core
  • 6gb RAM (2gbx2+1gbx2) * I will add 4GB x2 RAM (Will have 12gb total)
  • GeForce GT 220 video card
  • PSU 350w, but Im buying a 750w modular corsair cs PSU

If you need anymore info pls ask, I'd appreciate the help. Also, I am willing to spend 200-400 Euros. 

I'd like to get a current card but I just don't know what will work in my system in terms of compatibility and size.

a gtx 960 will be your best bet and a 500W PSU would be a better bet 

If that is the range here hell no. forget the GTX 960 is a a mistake at that price. R9 280x all the way at this price.

But yes a 500W would be more advisable.

EDIT: for 400 euro you can have an R9 290 easy. Look into the XFX 290 and if they are in range the Sapphire Tri-X 290.