Hi mate the best fastest more powerful one is the ASUS Radeon HD7850 DirectCU II TOP however i am not sure who might have that card in stock so if you can't find it you'll half to settle for second best which is GPU:MSI R7850 Power Edition Radeon HD 7850 Video Card - 2GB, GDDR5, PCI-Express 3.0(x16), 1x Dual-link DVI-I, 1x HDMI, 1x Mini DisplayPort, DirectX 11, Afterburner, Overclocking
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2886170&CatId=7387 <----- LINK TO THE BEAST!
TECHPOWERUP! http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CEgQFjAA&url=http%3A%2F%2Fwww.techpowerup.com%2Freviews%2FMSI%2FHD_7850_Power_Edition%2F&ei=K571T7-SJuPO2wXgm6nsBg&usg=AFQjCNFRuAhcS_s-g-1BmcQfRz_xydvIeg&sig2=xXll3QmG4wChUGTdE4xysg <-----PROFESSIONAL REVIEW/W BENCHMARKS