I have a D300, an SB-800, a standard zoom (Tamron 17-50 VC) and a tele zoom (Sigma 50-150). I was choosing between getting a second flash or a second body, either of which are available for a little more than $200.
For the second flash, I was thinking of the SB-700 ($329). Actually it would be my third flash (if I include the all-manual Yongnuo YN-560), but only my second CLS flash. I wanted to use the SB-700 as a commander because it has the new interface that allows easily switching from regular TTL to commander mode. For multiple flash setups, having another CLS flash would allow remote adjustment of intensity and mode (manual or TTL). I'm also curious about the SB-700's ability to do quick wireless ratios a la Canon.
If I wanted to save some money over the SB-700 and have a bit more power, I can also choose the SB-600, which I'm worried may soon disappear.
On the other hand, with a second body, I could use both of my lenses without having to switch them constantly. In addition, if I chose a lower-end body with automatic modes, it would be easier for my wife to use the camera.
The choice between the two came down to determining whether I wanted to do more setup shots (in which case the flash would be more useful) or continue working on covering events and candid shots (in which case the second body would be more important). I finally chose the latter.
Although I was interested in choosing a newer body with a better sensor, I chose a D70 because of the 1/500 sync speed and because it's much cheaper than a newer body. I got mine on ebay. Buying a used camera from ebay is not ideal and is more risky, but the seller was offering a 30-day warranty. It will be here by the end of the week - hopefully it performs as well as I expect.