I suppose an obvious question is: have you turned up your graphics settings since installing your new card? The game takes a significantly higher time to load levels on Ultra than it does on Medium - this is just part and parcel of the game's detail. Upgrading your graphics card, then, doesn't just mean more in-game FPS... it can mean slower loading times, for all of that eyecandy!
Though, hardware wise, I found upgrading from 4->8Gb of RAM helped nicely to give me a nice memory overhead. I think RAM and hard-drive speed are the two most important pieces of hardware in loading a map.