The Nvidia Geforce 8800 GT is a great single-card solution for $250. The GTX is even better but is a tad overpriced. Sell your GTS and grab a GT - trust me, the GT is more powerful than the GTS.

You've got a nice processor though.

Also there is no single card that can run Crysis on max possible detail without lag at any resolution above 1024x768 (maybe 1280x1024 as well). Even dual 8800 GTX's experienced lag at 1920x1200 on full settings. We all want the best possible visuals in Crysis, but for now, just stick with the not-so-high settings, especially when you get to multiplayer.