So which flash do you recommend for my Nikon D90 and why?
And also is flash used only for dark indoor situations? How many flashes to start out with and how much are they approximately?
Will the flash come with the wireless trigger?
Thanks!
Ahh, a D90. See, that adds a whole new dimension. Your D90 actually does not need a wireless trigger to control the SB600, 700, 800, or 900. It can do it with it's built in flash. In any case, I wasn't even really referring to an off-camera flash. More a flash that you can mount on your hot shoe, and then point it in the direction that you want the light to bounce off of. In any case, an SB-600 is around $200, I think the 700 is around 3 something, and the SB-800 can usually be found in the $320 range. The SB900 is a bit higher.
My recommendations, if your budget is around the price of the 50mm 1.4, would be the 50mm 1.8 and an SB600. I've actually assisted in a few weddings with the D90, 50mm 1.8 and SB600.
My gear has advanced significantly since then, but it's a very capable combo in just about any circumstance. It allows you off camera flash, a fast prime, on camera flash with bounce capabilities, and is a great bang for the buck.
As Derrel pointed out, the new 50mm 1.8 at around $220 actually focuses faster than the 50mm 1.4, which may be a consideration for you.
If budget was no concern, the 50mm 1.4 and SB900 are better choices. You really have a lot of options. Honestly, none of them are bad choices.
What is your total budget? From there, I could tell you what I would spend it on, knowing what I know now. FWIW, I advanced along the exact same upgrade path you are on now.
If you let us know the type of photography you primarily do, that would help, also. Indoors, outdoors? Moving subjects? Landscapes? People?