Studio XPS 8100, upgrade graphics

I have a Studio XPS 8100 I bought early 2011. I have used this as a gaming machine and it's listed me this long.  It has an I7 core,  8 GB RAM DDR3, and a radeon 5700. Is been crashing on WOW and it's getting upsetting,  I don't have the funds to but a new machine,  what's the best graphics card I can put in it before I push the CPU too hard?  (I plan on getting more RAM here soon)