D3 all the way. I went from a D200 to a D3 and although I had to spring for a bunch of lenses to go with it, the upgrade was definitely worth it. I had a pain in the wallet for a while but no regrets.
The high ISO performance has made possible a lot of shots that I couldn't do with the older technology. Even though the high ISO performance wasn't the original idea for the upgrade it has proven to be the largest benefit for me. Lots of other features of the D3 have also been worth using.
When the D3S came along I considered it, but to me it was a marginal upgrade so I didn't. Similarly the D4. The D4 is reputed to have better autofocus but that isn't a problem for me. So I can wait for the D5.
I'm now driven by ISO. Others are driven by pixels, so that's why the D800 has so much attention.