I've been using a dual screen set up for some time and I don't get entirely satisfied with the performance. My hardware is:
Core 2 Duo E6750
4GB ram
ATI Radeon HD3850 with 256MB
32" screen at 2560x1600
24" screen at 1920x1080

Using free divers the setup is usable but annoyingly lagged. The start menu isn't smooth when you press on it, when you move the windows is a bit laggy, or when you alt+tab, the desktop gets even unresponsive for a second, terribly slow.

I thought that maybe it was a matter of ati drivers problem, so I took an old nVidia GeForce 8400gs and with free or proprietary drivers the experience was even worse. Slower and choppier.

Any idea?
My first guess is that I need a new well compatible graphic card (nVidia?) but still not quite sure. Is that the problem? If it is, what would you recommend me? I'm not a gamer.