You want lots of megapixels when you plan to use an output medium that requires high resolution. Printing very large prints is the most common example. But if your print needs won't normally exceed 8x10 or 8x12, five or six megapixels is more than enough. The quality of an 8x10 print from a 5-MP source image can be outstanding.
I haven't seen a direct comparison of a print that size from a D700 or D3 in DX crop with an equivalent print from a D300. It would be interesting to see. I suspect the FX sensor would still provide a better result even though having lower resolution, especially at high ISO.
"Having been excited and sure i was going to get the D700 the now cheap D300 is a bargain!"
I think the D300 is a bargain even if there were no FX bodies! If I didn't already have a D2X, I'd get a D300 for the DX end of my kit.