Hi Eddie,
Off camera flash can be tricky to set up and there's many ways to go about it, but it's the most fun I've had with my camera in a long time so worth it!
You need to have a way of triggering the flash firstly. You can do this by having a flashgun mounted on the top of your 40D (such as your 550ex) that triggers the flash, however they it wouldn't be off camera and you'd need
another flashgun to act as the off camera bit.
The advantage to this is that you can retain the ability to use auto-power (TTL) flash where it will meter the flash power to produce a more desirable result without fiddling with manual settings.
The other option is to use radio triggers. They come in two parts, one that mounts on your hotshoe (transmitter) and one that sits on the flashgun's hotshoe (receiver). It works very well but you can only use them on manual.
With regards to the question about firing a camera wirelessly and then having OCF as well, it can be done and I tested this out a few weeks ago myself.
Basically I did this with radio triggers, I'll try to explain:
Take a radio receiver and attach it to camera as shown in
this picture (transmitter/trigger is on the right, receiver on the left). This shows the transmitter being used as a manual button/finger triggered device that will activate the shutter.
Then take another transmitter and fit it on the hotshoe of your camera
like this, with the receiver on the flashgun
like this.
The one problem you have to overcome is that when you press the handheld trigger, it will set off the shutter button and the flashgun, and they will not be in sync. To get around this, you need to fiddle with the channel switches on the underside of one of those pairs of transmitters/receivers to a different channel.
In brief, the order of events will be like this:
Channel 1
Handheld Transmitter > Receiver > Trigger shutter
Channel 2
Trigger shutter > hotshoe transmitter > flash receiver
Please ask if some/most/all of that doesn't make sense!
