Here are the basics you will need:
When looking for a flash try and find one that has manual control and external sync ports for hooking up to your remotes/receivers. Since we are using these off camera, brand is really a non issue. You can use Canon flashes with Nikons and vice versa. I suggest getting older Nikon speedlights (SB-24, SB-26, 28, 80dx etc) that can be found used for around $100 because they have both of the features mentioned above. No sense spending big bucks on a 580EX or SB900 when we won't be using half the features. (ettl and ittl)
If you get a SB-26, they will have a built in optical trigger. Another, line of site triggering option.
I like the Wescott 8 foot stands. You can find them on BH Photo for $34 a piece here. These fold down small and hold 5lbs.
These are the interface between your flash and light stand. $25 here.
I have both the white shoot through and silver reflective umbrellas, but tend to use the shoot through more. You can find them here and here. $19.50 each
Industry standard here are the Pocket Wizards They transmit and receive up to 1600 feet and are rock solid. And cost $169....each. One for your camera, and one for each flash.
Yikes! Luckily for us there are alternatives.
I've had great success with my RF 602's. They trigger over 400 feet and I've never had one fail to fire (within those distances). They are hot shoe adaptable so if your flash doesn't have the sync ports mentioned above, you can still trigger them! How great is that! And heres the kicker...the cost $58 for one transmitter and 2 receivers!! So for less than half the cost of 1 pocket wizard you can be up and running with these gems and have cash left over to buy another flash...or two. You can find these all over Ebay and come with cords specific to your camera. They can remotely fire your shutter also. I would suggest you get them from Amazon if you want to get them delivered in a timely fashion.
Next we'll look at other light modifiers and misc bits to keep you clicking! Stay tuned....