Without knowing what you want to use the flash for (what sort of situation, what sort of photography) it is really difficult to zoomed down to what flash you might need.
Take note of the following,
1) Controls (buttons layout, ergonomic, etc)
2) Budget (you already have this, so no worries)
3) Weigh (SB900 is not exactly a light weigh flash, and your D90 is not exactly a light camera too).
4) Size of the flash, sometime smaller flash is more desirable. In the past, I am using a 430EX II for my camera, but nowaday I have settle for a 270EX which I find it good enough, small enough and light enough for whole day of shooting.
Also do look at 3rd party flashes such as the Yongnuo and Nissins. They are very good flash and very cheap too.
In theory... yes. But also don't forget this saying, "Only buy those that you need."
Like I said before, weight is a limiting fact too, not just budget. SB900 or SB700 are not very light in terms of bulk and weight... Sometime it would be good to go for lighter product so it will not put a strain on your hands...