I fully seated it in the PCI-E slot.
I connected both 6-pin connectors and the card power connector LEDs are green, showing they're getting the required power.
I start Diablo and immediately throw the game into the highest settings:
1920x1080 Resolution, Full Screen
Texture, Shadow, Physics and Clutter: High
Vertical Sync: On and Off (Tried both)
Low FX - Off
I load into the game and run around... and man, am I disappointed. First, the UI is very slow to load in. Second, while the game looks good, the FPS is low and everything feels sluggish.
I've uninstalled all old Nvidia drivers and re-installed 301.42, along with all the other software in the suite.
I went into the Nvidia control panel and set all the advanced options to Default and picked 'Quality' for the performance mode.
CPU: AMD Phenom II X4 970 Black Edition 3.98Ghz
PSU: OCZ ZT 750W
GPU: Asus GTX 670
Mobo: Asus M5A99X Evo
HD: 1TB Hitachi 7200RPM
Ram: G.Skill 1333 DDR3 (2x4GB)
What am I doing wrong and/or missing?