I can't believe I forgot to address the video cards. -_-
The 9600 GT is basically a 9800 GTX with half of the SPs and lower clock speeds. It performs surprisingly close to an 8800 GT, but there's still a difference.
The 8800GS is another story. It's a 9800 GTX with one forth of the SPs disabled(Thus giving it a more powerful chip than the 9600 GT), but the memory speeds are cut by quite a bit(They partially disabled the memory bus.), and there's not as much ram. It performs very much like the 9600 GT. Hell, even nVidia recognizes this. they renamed it to the 9600 GSO a few weeks ago.
As for which one is better between the two, it depends. If you're going to be using a lot of AA, I'd go with the 9600 GT. If you are going to be playing a lot of shader intensive games(BioShock/Gears of War/Crysis/etc), the 9600 GSO/8800 GS might be a better choice. It's easier to find the 9600 GT for a good price, however, and the 9600 GSO/8800 GS isn't that much faster in shader intensive stuff.
The 8800 GT is better than those two, but not by a huge ZOMG difference. If you run stuff at high resolutions(1650x1050 and higher), then you'll want the 8800GT or higher.
Now, if you really want a mega card, just wait until next month. nVidia is going to release a new Mega fast card then. It'll be a one card solution as well, which means you won't run into the problems that SLI can have(some games have problems, or won't get much faster with it on, or will get slower, yes, SLOWER, with it on, some games will need updated drivers in order for SLI to work with it, etc).