FiringSquad has a nice video card review for anyone who cannot decide on whether they want to go with two GeForce 8800GTS’s running in SLI or a single 8800GTX.
At my preferred resolution of 1920×1200 their benchmarks show the following FPS:
Call of Duty 2 GTX:56 2xGTS: 67.2
Half Life 2 Lost CoastGTX: 88.8 2xGTS: 110.
Company of Heroes GTX:83.9 2xGTS: 117.6
Quake 4 GTX: 84.5 2xGTS: 113.6
Visit the site to see the rest. But what you can see above is a clear improvement when choosing to go the SLI GTS route versus the lone GTX. This is not a end all on the topic though, its only one angle. There are many other factors to consider such as SLI incompatibility’s, Price, number of DVI interfaces, plus more.
In my opinion you should always go for more video cards because you get more DVI interfaces, but I guess writing for MultiMonitor.net does not make me impartial.