Yeah, the SB-26 and 28 would fit on the D90. General rule of thumb, if Nikon makes it, it's safe to put on your Nikon camera. If not, don't.
With flashes and hot shoes, different brands may have different voltages. If you use the wrong flash with your camera, it could fry your body, your flash, or both. Most likely, just your body.
The other thing about flash photography is that it seldom looks really good when used on the camera's hot shoe. Sure, you'll look like a professional with a big towering flash above your camera, but the photos will be unlikely to look all that great.
Most of the time, you'll want to use off-axis lighting. This means your flash is not on your camera. There are many ways to trigger an off-camera flash, ranging from free to expensive.
The SBs have built-in optical triggers. This means that when it sees another flash, it'll fire. You can stop down the flash built in to your D90 by putting it to manual and choosing power, i.e. M, 1/2, 1/4... etc. If you choose the correct number, it'll trigger your SB without affecting your photo much at all.
Other ways to do it range from PC cords, for which you'll need the right plugs (which are infinitely more complicated than they have to be). Most often, it's the most financially practical option. The SBs have a Female PC connection, so you'll need a cable with the male connection on that end. As for the camera end, some cameras have the hole for a PC cord, and others don't. If it doesn't, you can buy an adapter that goes onto the hot shoe and does its job.
Sync cords are incredibly reliable, because there is a physical connection from the camera to the flash, but they are limited in their range. Not to mention that PC cords are expensive, for wires with bits of metal on the end, and like all other cables, they
are prone to breaking.
Some manufacturers have begun switching to standard Audio jacks, which are a whole lot easier to find, cheap. If you look hard you can find them for $1 for 25 feet. It's nutz. To use a 3.5mm Audio cord you'll need the right adapters- one for your camera, one for your flash. It's the better way to go, and I, like many others, am waiting patiently until it is a standard hole on every camera and every flash.
The more expensive way to trigger your flash is a wireless trigger- most reliably in the form of the Pocketwizard. I don't have radio triggers, but they range from the super-cheap, made-in-china sold-on-ebay type (often, they work for a bit then die, or end up being unreliable. Hence the name "newb tax", because everyone who doesn't get wonderful guidance like this spends money on them before getting something good) to the expensive, reliable, and long-lasting ones. Paul C Buff Cybersyncs are a good middle option- they're relatively cheap, reliable, and built pretty well.
As you go up in price, you'll be able to change your flash's settings from your camera, which is useful if your flashes are, say, hundreds of feet away. That said, your flash won't work hundreds of feet away unless you've put down the money for a good trigger with a nice, long range.
All of this information, and considerably more, is available on
www.strobist.com. Strobist is a blog regarding the use of flashes (or strobes) in photography.
Start with Lighting 101 and work your way through. Soon enough, you'll be making posts like these on Newschoolers.
