Ex-nvidia fanboy here who's now an ATI convert (the geforce 5's were just too crappy). In my (limited) experience, ATI cards are just physically higher quality, heck even the PCB is stiffer. I guess it depends on the brand though, there are some pretty cheap nvidia manufacturers out there.
Yeah Nvidia rules the low-end, great :) If you're a gamer though, ATI. 9800 pro is down to ~US130 shipped. X800XL is another unreal value - it outperforms a 6800GT on anything that's not Doom3 or OpenGL, and it's ~US80 cheaper. X850XT outperforms a single 6800 Ultra, it's reportedly quieter, and it's ATI :) If you're in the ~150-200 price range, then yeah go for a 6600GT, they're pretty cool for the price. That's the only nvidia card I would buy right now.
SLI is only worth it if you have to have the fastest thing now, and want to get dual 6800GT's or Ultras. But, next-gen cards will likely outperform that setup with a single card. Nvidia is very smart - "a fool and his money are easily parted." Like Iggy said, as long as it plays your games. That's the smartest way to go.