Heartyfisher - I am satisfied with my D700 (kind of have to be since it's got the best low light sensor around). You're right that I might not be satisfied with a D3X if it has more noise than the D700. I really can't say enough good about the D700 - I do need to cough out the cash to get the battery grip to help with sore wrist/arm issues though. When I looked at my first shot with it on the LCD screen I said, "oh now I get it, that's how the pros are doing it". It was night and day. I do wonder if my D80 had some focus accuracy issues though.
The D300 is incredible too at low ISO. I can't really see the difference between that and the D700 in good light. The D300 is also extremely fun for sports and wildlife.
Anyone considering a D300 of less, just go ahead and get the D300 - unless of course its replacement has been announced. I wouldn't get anything less if you can afford it and are or will be a serious photographer. The focus system alone makes it worth it. No camera takes good pictures when they aren't in focus.