robertperry: BULL!!! But still one route to take. Have you forgotten the initial question? Shoot small wedding for a friend.
If you have a Nikon Canon flash that is older, and an adapter to your camera fine. Other brands of flash like vivatar work well. Consider that the circuit is basically the same. Yes the flash that is much more expensive and the camera communicate, but it is certainly not necessary to have that complication. With simple math you can find the f stop and pre plan you stage shots. ,,, goal is not to over expose or have harsh light. Not complicated. (unless you have to decide which flash focus setting, power out put to select etc on a very expensive complicated flash unit.
Like in a space shot ,,, redundancy is a safe goal. If equipment fails grab a backup. Most people do not look at auxiliary equipment brand names. Vivatar etc means nothing to most of the public. Camera perhaps, Canon and Nikon names and reputations are great, but other cameras also take great shots and then ask yourself, what are the shots to be used for. Most people adding friends shot use P&S cameras, perhaps even a 3 meg pix. Still good photos for slide show or even an 8x10. Remember people do not use an eye lope to examine for pixes.
When starting out use KISS, keep it simple shooter approach. Also do not invest great bucks for starting out. Recall All: the original question was about shooting a friends small wedding....
If you have the big $ to waste then by all means go to a photo equipment shop and have a 18 y/o novices tell you all about the virtues of the expensive equipment!! If you want want to really know about the equipment check the specifications and also educate your self regarding the circuit designs used for flashes and control units. I did not say design them, rather read about them.
Basically flashes have a charging circuit a trigger voltage and a flash tube. The flashes that communicate with cameras or with signals they pick up. The communicating ones are more complicated in the trigger circuit that regulates the duration of the flash and the power.
Personally I would rather have the versatility and reliability of 3 guide number 40 power than one guide 120. I use 2 PVC black 3/4 flash stands to give 45 degree fill along with my on camera flash and perhaps even a bracket flash off side of camera. All are about guide 40 and not expensive. Example Vivitar Sf-4000 Auto Bounce Zoom Slave Flash with Bracket $25 online ,, try one, read comments.
http://www.google.com/products/catalog?oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&q=low+cost+vivitar+flash&um=1&ie=UTF-8&tbm=shop&cid=5793476743257228559&sa=X&ei=GKt2TujXFJG4twewncSYCg&ved=0CIUBEPMCMAQSimplicity relates to your ability to control and the reliability of the equipment. When shooting, you do not have time to play with complicated controls. To be a good candid photographer at weddings you will be a Quick Draw McGraw.
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http://en.wikipedia.org/wiki/Quick_Draw_McGraw)