One GTX would be better than two GTSs running in SLI, especially if the most you'll be using is 1024x768. They're GTSs... a GTX will always look better. Adding two GTSs together will not make them capable of what a GTX is capable of, because that's not how SLI works.
Now, if you were to have a much higher resolution, say one that a single GTX (or even a single GTS for that matter) would not be enough for, then running an SLI set-up would help. As I understand it, this is because rather than having to display an image in a high resolution (i.e. a large monitor screen) which it could have difficulty doing, in SLI each card only has to display an image for a portion of that resolution (one half of the screen). In other words, in SLI the workload is split up between the two cards, but performance (visual quality) is not improved.
As for SLI/Crossfire vis a vis dual-core processors, you would be comparing apples to oranges. A dual-core/multi-core processor should only be compared to single-core processors. For gaming, a dual-core processor would help if you want to have other programs running while you are playing a game (e.g. anti-virus, another game, listening to music, etc.). Future games (and a few current ones) will also make use of dual-core.
