Only reason I mentioned this is because had similiar issues where I would get sudden drops in fps from 100 down to 30-40 fps or even when capped at 60 fps would drop down to 20-30 fps. This would affect my gameplay and drove me crazy for several weeks. It was a Nvidia driver issue that has only been recently corrected by the beta drivers and also involved changing adaptive vsync settings. if this isn't your problem then sorry for posting, just trying to help troubleshoot, and you won't find me on your threads any more. if it's an overall performance issue then this may not be the problem.
I'm using dual 27" monitors at 1980 x 1080, using a 256gb mSATA SSD dedicated for gameplay with a 2GB GTX 660ti. I used a 2GB GTX 660 for a while and it worked fine as well. I have read the Dell GTX 660 is a 1.5gb card.
Thread is here:
http://en.community.dell.com/support-forums/desktop/f/3515/t/19474677.aspx